posix: implement shm_unlink()

[ycsin]

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

shm_unlink - remove a shared memory object (REALTIME)

The shm_unlink() function shall remove the name of the shared memory object named by the string pointed to by name.

If one or more references to the shared memory object exist when the object is unlinked, the name shall be removed before shm_unlink() returns, but the removal of the memory object contents shall be postponed until all open and map references to the shared memory object have been removed.

Even if the object continues to exist after the last shm_unlink(), reuse of the name shall subsequently cause shm_open() to behave as if no shared memory object of this name exists (that is, shm_open() will fail if O_CREAT is not set, or will create a new shared memory object if O_CREAT is set).

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

See RFC #51211 for more info.