`child::terminate()` result in ECHILD in `on_exit` since boost 1.78 including 1.84

## Description

In boost 1.77 the simple program below results in

> Child completed with exit code=9, and error code: Success

whereas since boost 1.78 (also tested 1.79, 1.80, 1.83 & 1.84) it results in

> Child completed with exit code=-1, and error code: No child processes

Only boost was changed for these tests compiled using gcc 13.2.0, but has been also been confirmed on other Linux systems with kernel version 5.15 and 6.2.9.

## Test program

```c++
#include
#include

#include

int main() try {
boost::asio::io_context ctx;
namespace bp = boost::process;
auto proc = bp::child(
bp::argv({"sleep", "5"}),
ctx, bp::on_exit = [](int exit, const std::error_code &ec) {
std::cerr << "Child completed with exit code="
<< exit
<< ", and error code: " << ec.message() << '\n';
});

proc.terminate();

// Run to completion
ctx.run();

} catch (std::exception const &e) {
std::cerr << "Unexpected exception " << e.what() << '\n';
}
```

## Syscalls

If it help I'm including the resulting syscalls for both cases.

In boost 1.78+ it seems to `waitpid()/wait4` a second time. Section extracted here:

```
...
[pid 1387918] wait4(1387919,
[pid 1387919] +++ killed by SIGKILL +++
<... wait4 resumed>[{WIFSIGNALED(s) && WTERMSIG(s) == SIGKILL}], 0, NULL) = 1387919
...
wait4(1387919, 0x7ffe02136250, WNOHANG, NULL) = -1 ECHILD (No child processes)
```

### boost 1.77 -> OK

```console
$ strace -f -e '!%file,%memory' result/bin/test
arch_prctl(0x3001 /* ARCH_??? */, 0x7ffc5bd058b0) = -1 EINVAL (Invalid argument)
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\220\202\2\0\0\0\0\0"..., 832) = 832
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7f225d6c6740) = 0
set_tid_address(0x7f225d6c6a10) = 1388959
set_robust_list(0x7f225d6c6a20, 24) = 0
rseq(0x7f225d6c7060, 0x20, 0, 0x53053053) = 0
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
futex(0x7f225d65e6fc, FUTEX_WAKE_PRIVATE, 2147483647) = 0
getrandom("\x9f\x8c\xa0\xb6\x55\xec\x3d\x08", 8, GRND_NONBLOCK) = 8
eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK) = 3
epoll_create1(EPOLL_CLOEXEC) = 4
timerfd_create(CLOCK_MONOTONIC, TFD_CLOEXEC) = 5
epoll_ctl(4, EPOLL_CTL_ADD, 3, {events=EPOLLIN|EPOLLERR|EPOLLET, data={u32=37285688, u64=37285688}}) = 0
write(3, "\1\0\0\0\0\0\0\0", 8) = 8
epoll_ctl(4, EPOLL_CTL_ADD, 5, {events=EPOLLIN|EPOLLERR, data={u32=37285700, u64=37285700}}) = 0
pipe2([6, 7], 0) = 0
fcntl(6, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
fcntl(7, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
fcntl(6, F_SETFD, FD_CLOEXEC) = 0
fcntl(7, F_SETFD, FD_CLOEXEC) = 0
epoll_ctl(4, EPOLL_CTL_ADD, 6, {events=EPOLLIN|EPOLLPRI|EPOLLERR|EPOLLHUP|EPOLLET, data={u32=37285888, u64=37285888}}) = 0
rt_sigaction(SIGCHLD, {sa_handler=0x407a70, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7f225d172eb0}, NULL, 8) = 0
pipe2([8, 9], 0) = 0
fcntl(9, F_SETFD, FD_CLOEXEC) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f225d6c6a10) = 1388960
strace: Process 1388960 attached
[pid 1388959] close(9) = 0
[pid 1388960] set_robust_list(0x7f225d6c6a20, 24
[pid 1388959] read(8,
[pid 1388960] <... set_robust_list resumed>) = 0
[pid 1388960] close(8) = 0
[pid 1388959] <... read resumed>"", 8) = 0
[pid 1388959] close(8) = 0
[pid 1388959] wait4(1388960, 0x7ffc5bd05754, WNOHANG, NULL) = 0
[pid 1388959] kill(1388960, SIGKILL) = 0
[pid 1388959] wait4(1388960, 0x7ffc5bd05754, WNOHANG, NULL) = 0
[pid 1388959] epoll_wait(4, [{events=EPOLLIN, data={u32=37285688, u64=37285688}}], 128, 0) = 1
[pid 1388960] +++ killed by SIGKILL +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=1388960, si_uid=1337, si_status=SIGKILL, si_utime=0, si_stime=0} ---
write(7, "\21\0\0\0", 4) = 4
rt_sigreturn({mask=[]}) = 1
wait4(1388960, [{WIFSIGNALED(s) && WTERMSIG(s) == SIGKILL}], WNOHANG, NULL) = 1388960
write(2, "Child completed with exit code=", 31Child completed with exit code=) = 31
write(2, "9", 19) = 1
write(2, ", and error code: ", 18, and error code: ) = 18
write(2, "Success", 7Success) = 7
write(2, "\n", 1
) = 1
epoll_ctl(4, EPOLL_CTL_DEL, 6, 0x7ffc5bd05704) = 0
close(6) = 0
close(7) = 0
rt_sigaction(SIGCHLD, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f225d172eb0}, NULL, 8) = 0
close(4) = 0
close(5) = 0
close(3) = 0
exit_group(0) = ?
+++ exited with 0 +++
```

### boost 1.84 -> ECHILD

```console
$ strace -f -e '!%file,%memory' result/bin/test
arch_prctl(0x3001 /* ARCH_??? */, 0x7ffe02136880) = -1 EINVAL (Invalid argument)
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
close(3) = 0
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\220\202\2\0\0\0\0\0"..., 832) = 832
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7ffad927f740) = 0
set_tid_address(0x7ffad927fa10) = 1387918
set_robust_list(0x7ffad927fa20, 24) = 0
rseq(0x7ffad9280060, 0x20, 0, 0x53053053) = 0
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
futex(0x7ffad925e6fc, FUTEX_WAKE_PRIVATE, 2147483647) = 0
getrandom("\xf2\xaf\x43\xf5\x80\x83\xac\x46", 8, GRND_NONBLOCK) = 8
eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK) = 3
epoll_create1(EPOLL_CLOEXEC) = 4
timerfd_create(CLOCK_MONOTONIC, TFD_CLOEXEC) = 5
epoll_ctl(4, EPOLL_CTL_ADD, 3, {events=EPOLLIN|EPOLLERR|EPOLLET, data={u32=25309008, u64=25309008}}) = 0
write(3, "\1\0\0\0\0\0\0\0", 8) = 8
epoll_ctl(4, EPOLL_CTL_ADD, 5, {events=EPOLLIN|EPOLLERR, data={u32=25309020, u64=25309020}}) = 0
pipe2([6, 7], 0) = 0
fcntl(6, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
fcntl(7, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
fcntl(6, F_SETFD, FD_CLOEXEC) = 0
fcntl(7, F_SETFD, FD_CLOEXEC) = 0
epoll_ctl(4, EPOLL_CTL_ADD, 6, {events=EPOLLIN|EPOLLPRI|EPOLLERR|EPOLLHUP|EPOLLET, data={u32=25309200, u64=25309200}}) = 0
rt_sigaction(SIGCHLD, {sa_handler=0x4073f0, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7ffad8d72eb0}, NULL, 8) = 0
pipe2([8, 9], 0) = 0
fcntl(9, F_SETFD, FD_CLOEXEC) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7ffad927fa10) = 1387919
strace: Process 1387919 attached
[pid 1387918] close(9) = 0
[pid 1387919] set_robust_list(0x7ffad927fa20, 24
[pid 1387918] read(8,
[pid 1387919] <... set_robust_list resumed>) = 0
[pid 1387919] close(8) = 0
[pid 1387918] <... read resumed>"", 8) = 0
[pid 1387918] close(8) = 0
[pid 1387918] wait4(1387919, 0x7ffe02136364, WNOHANG, NULL) = 0
[pid 1387918] wait4(1387919, 0x7ffe021366f4, WNOHANG, NULL) = 0
[pid 1387919] arch_prctl(0x3001 /* ARCH_??? */, 0x7ffda4ad19d0
[pid 1387918] kill(1387919, SIGKILL
[pid 1387919] <... arch_prctl resumed>) = -1 EINVAL (Invalid argument)
[pid 1387918] <... kill resumed>) = 0
[pid 1387918] wait4(1387919,
[pid 1387919] +++ killed by SIGKILL +++
<... wait4 resumed>[{WIFSIGNALED(s) && WTERMSIG(s) == SIGKILL}], 0, NULL) = 1387919
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=1387919, si_uid=1337, si_status=SIGKILL, si_utime=0, si_stime=0} ---
write(7, "\21\0\0\0", 4) = 4
rt_sigreturn({mask=[]}) = 1387919
epoll_wait(4, [{events=EPOLLIN, data={u32=25309008, u64=25309008}}, {events=EPOLLIN, data={u32=25309200, u64=25309200}}], 128, -1) = 2
read(6, "\21\0\0\0", 4) = 4
read(6, 0x7ffe0213651c, 4) = -1 EAGAIN (Resource temporarily unavailable)
epoll_wait(4, [], 128, 0) = 0
wait4(1387919, 0x7ffe02136250, WNOHANG, NULL) = -1 ECHILD (No child processes)
write(2, "Child completed with exit code=", 31Child completed with exit code=) = 31
write(2, "-1", 2-1) = 2
write(2, ", and error code: ", 18, and error code: ) = 18
write(2, "No child processes", 18No child processes) = 18
write(2, "\n", 1
) = 1
epoll_ctl(4, EPOLL_CTL_DEL, 6, 0x7ffe021366c4) = 0
close(6) = 0
close(7) = 0
rt_sigaction(SIGCHLD, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7ffad8d72eb0}, NULL, 8) = 0
close(4) = 0
close(5) = 0
close(3) = 0
exit_group(0) = ?
+++ exited with 0 +++
```