spawn:posix: Take care of POSIX semaphore "shared" semantics.

* src/spawn-posix.c (spawn_detached): Don't call _gpgrt_post_syscall
on the child process.
(_gpgrt_process_spawn): Likewise.

On NetBSD, a POSIX semaphore is somehow shared among processes, even
if sem_init is called with PSHARED=0.

• GnuPG-bug-id: T8065
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>