nptl: Set sem_open as a non cancellation point (BZ #15765)

This patch changes sem_open to not act as a cancellation point.
Cancellation is disable at start and reenable in function exit.
It fixes BZ #15765.

Tested on x86_64 and i686.

	[BZ #15765]
	* nptl/Makefile (tests): Add tst-sem16.
	* nptl/tst-sem16.c: New file.
	* nptl/sem_open.c (sem_open): Disable asynchronous cancellation.