Cleanup sigpause implementation

This patch simplify sigpause by remobing the single thread optimization
since it will be handled already by the __sigsuspend call.

Checked on x86_64-linux-gnu.

	* sysdeps/posix/sigpause.c (do_sigpause): Remove.
	(__sigpause): Rely on __sigsuspend to implement single thread
	optimization.  Add LIBC_CANCEL_HANDLED for cancellation marking.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Zack Weinberg <zackw@panix.com>