posix: implement pthread_atfork()

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, pthread_atfork() is one of them.

pthread_atfork - register fork handlers

The pthread_atfork() function shall declare fork handlers to be called before and after fork(), in the context of the thread that called fork(). The prepare fork handler shall be called before fork() processing commences. The parent fork handle shall be called after fork() processing completes in the parent process. The child fork handler shall be called after fork() processing completes in the child process. If no handling is desired at one or more of these three points, the corresponding fork handler address(es) may be set to NULL.

If a fork() call in a multi-threaded process leads to a child fork handler calling any function that is not async-signal-safe, the behavior is undefined.

The order of calls to pthread_atfork() is significant. The parent and child fork handlers shall be called in the order in which they were established by calls to pthread_atfork(). The prepare fork handlers shall be called in the opposite order.

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_atfork.html

See RFC #51211 for more info.