[v2][Linux] process_is_running() returns false with musl libc

**Observed with boost-1.81.0**

On Linux, the default _exit code_ (`detail::still_active`) is defined as `0x7f`. But on systems with musl libc (e.g. Alpine Linux and some OpenEmbedded/Yocto variants), `WIFSIGNALED(0x7F)` results in a non zero value. This causes a wrong return value in `process_is_runing()`:

```
inline bool process_is_running(int code)
{
return !WIFEXITED(code) && !WIFSIGNALED(code);
}
```

When `process_is_running()` is called with the initial value (`detail::still_active`) of `basic_process::exit_status_` (via `async_wait_op_::res` in `async_wait_op_::operator()()`) , this leads to premature completion (and termination) of a newly created process.

**Test program:**
```
#include

#ifndef __GLIBC__
/* close_range() hasn't been accepted for musl-libc yet:
* https://www.openwall.com/lists/musl/2023/09/01/4
*/
int close_range(unsigned int, unsigned int, int);

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include
#include

int close_range(unsigned int first, unsigned int last, int flags)
{
return syscall(__NR_close_range, first, last, flags);
}
#endif

#include

#include
#include
#include
#include

int main(int, char **)
{
using namespace boost::process::v2;

boost::asio::io_context ctx;
async_execute(process(ctx, "/bin/echo", {"1", "2", "3", "4"}),
[](boost::system::error_code const &ec, int exitCode)
{
if (ec) {
std::cerr << "Executing process failed: " << ec << '\n';
return;
}

std::cout << "Process existed with status " << exitCode << '\n';
});

ctx.run();
}
```

Output _Glibc_ based systems (correct):
```
1 2 3 4
Process existed with status 0
```

Possible output in _musl libc_ based systems (not correct, may vary due to timing):
```
Process existed with status 127
```

Note that `127` here is the initial value of `basic_process::exit_status_` rather than the real exit code of `/bin/echo`. In my tests, the `echo` process is usually killed with `SIGKILL` before producing any output:

```
process::v2::detail::execute_op::~execute_op()
\-std::unique_ptr::~unique_ptr() [execute_op::proc member]
\-...
\-basic_process::~basic_process()
\-detail::basic_process_handle_fd<...>::terminate_if_running()
\-::kill(pid_, SIGKILL);
```