posix: implement `pthread_sigmask()`

[ycsin]

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

pthread_sigmask, sigprocmask - examine and change blocked signals

The pthread_sigmask() function shall examine or change (or both) the calling thread's signal mask, regardless of the number of threads in the process. The function shall be equivalent to sigprocmask(), without the restriction that the call be made in a single-threaded process.

In a single-threaded process, the sigprocmask() function shall examine or change (or both) the signal mask of the calling thread.

If the argument set is not a null pointer, it points to a set of signals to be used to change the currently blocked set.

The argument how indicates the way in which the set is changed, and the application shall ensure it consists of one of the following values:

SIG_BLOCK
The resulting set shall be the union of the current set and the signal set pointed to by set.
SIG_SETMASK
The resulting set shall be the signal set pointed to by set.
SIG_UNBLOCK
The resulting set shall be the intersection of the current set and the complement of the signal set pointed to by set.
If the argument oset is not a null pointer, the previous mask shall be stored in the location pointed to by oset. If set is a null pointer, the value of the argument how is not significant and the thread's signal mask shall be unchanged; thus the call can be used to enquire about currently blocked signals.

If there are any pending unblocked signals after the call to sigprocmask(), at least one of those signals shall be delivered before the call to sigprocmask() returns.

It is not possible to block those signals which cannot be ignored. This shall be enforced by the system without causing an error to be indicated.

If any of the SIGFPE, SIGILL, SIGSEGV, or SIGBUS signals are generated while they are blocked, the result is undefined, unless the signal was generated by the action of another process, or by one of the functions kill(), pthread_kill(), raise(), or sigqueue().

If sigprocmask() fails, the thread's signal mask shall not be changed.

The use of the sigprocmask() function is unspecified in a multi-threaded process.

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

See RFC #51211 for more info.