nptl: Add more coverage in tst-cancel4

nptl: Add more coverage in tst-cancel4

This patch adds early cancel test for open syscall through a FIFO
(thus makign subsequent call to open block until the other end is
also opened).

It also cleanup the sigpause tests by using sigpause along with
SIGINT instead of __xpg_sigpause and SIGCANCEL.  Since the idea
is just to test the cancellation handling there is no need to expose
internal glibc implementation details to the test through pthreadP.h
inclusion.

Tested x86_64.

	* nptl/tst-cancel4-common.c (do_test): Add temporary fifo creation.
	* nptl/tst-cancel4-common.h (fifoname): New variable.
	(fifofd): Likewise.
	(cl_fifo): New function.
	* nptl/tst-cancel4.c (tf_sigpause): Replace SIGCANCEL usage by
	SIGINT.
	(tf_open): Add early cancel test.