posix: implement pthread_testcancel()

[ycsin]

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

pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel - set cancelability state

The pthread_setcancelstate() function shall atomically both set the calling thread's cancelability state to the indicated state and return the previous cancelability state at the location referenced by oldstate. Legal values for state are PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DISABLE.

The pthread_setcanceltype() function shall atomically both set the calling thread's cancelability type to the indicated type and return the previous cancelability type at the location referenced by oldtype. Legal values for type are PTHREAD_CANCEL_DEFERRED and PTHREAD_CANCEL_ASYNCHRONOUS.

The cancelability state and type of any newly created threads, including the thread in which main() was first invoked, shall be PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DEFERRED respectively.

The pthread_testcancel() function shall create a cancellation point in the calling thread. The pthread_testcancel() function shall have no effect if cancelability is disabled.

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

See RFC #51211 for more info.

[cfriedt]

Cannot assign to @moonlight83340, but they would like to implement this one.

Thanks for helping out 🙏