boost::process::child::terminate() no longer works

This test will work with Boost 1.70+, but starting with 1.71 the process will still exist although `running()` says otherwise...

```
TEST(Process, test)
{
boost::process::child c(boost::process::search_path("sleep"), "60");
EXPECT_TRUE(c.running());
const pid_t pid = c.id();
c.terminate();
c.wait();
EXPECT_FALSE(c.running());
}
```

Adding `EXPECT_NE(0, kill(pid, 0));` to the test will highlight the issue.
Also using `kill(pid, SIGKILL)` explicitly in place of `terminate()` will get rid of the process.

The issue happens on CentOS 7, ubuntu 16.04 and ubuntu 19.10.