posix: implement sigaction()

[ycsin]

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

sigaction - examine and change a signal action

The sigaction() function allows the calling process to examine and/or specify the action to be associated with a specific signal. The argument sig specifies the signal; acceptable values are defined in <signal.h>.
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See https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaction.html

See RFC #51211 for more info.