errno is modified in signal handlers

[buchgr]

Using thread sanitizer I found that the signal handler for SIGCHLD processx__sigchld_callback

processx/src/unix/sigchld.c

Line 41 in 8a959af

wp = waitpid(ptr->pid, &wstat, WNOHANG);

may modify errno. The fix is to save errno when entering and restoring it when exiting the signal handler, like in processx.c

processx/src/unix/processx.c

Line 92 in b7e15fb

saved_errno = errno;

I am happy to send a PR, but it's unclear to me yet on how to best write an automated test for it.

[gaborcsardi]

Thanks! A PR is welcome. No need to test for it in the PR, but a good test would be to run thread sanitizer on GHA.