(use_sa_restart): New static variable.

(sig_on_signal): Add SA_RESTART if appropriate.
(oop_signal_new): Don't just blindly set FD_CLOEXEC and
	O_NONBLOCK.  Read the old settings first.
(oop_signal_use_sa_restart): New function.

Signed-off-by: Magnus Holmgren <magnus@kibibyte.se>