rt: Set shm_open as a non cancellation point (BZ #18243)

This patch changes shm_open to not act as a cancellation point.
Cancellation is disable at start and reenable in function exit.
It fixes BZ #18243.

Tested on x86_64 and i686.

	[BZ #18243]
	* rt/Makefile (test): Add tst-shm-cancel.
	* rt/tst-shm-cancel.c: New file.
	* sysdeps/posix/shm_open.c: Disable asynchronous cancellation.