* cygwait.h (enum cw_wait_mask): Add cw_sig_restart. Add comments

	* cygwait.h (enum cw_wait_mask): Add cw_sig_restart.  Add comments
	to explain the meaning of the possible values.
	* cygwait.cc (is_cw_sig_restart): Define.
	(is_cw_sig_handle): Check for cw_sig_restart as well.
	(cygwait): Restart always if cw_sig_restart is set.
	* thread.cc (pthread::join): Call cygwait with cw_sig_restart flag
	to avoid having to handle signals at all.