(sys_sig_owner): Added static qualifier. (use_sa_restart): New static...

(sys_sig_owner): Added static qualifier. (use_sa_restart): New static variable. (sys_on_signal): Use SA_RESTART if use_sa_restart is true. (oop_sys_use_sa_restart): New function. (sys_on_fd): Require the callback to be non-NULL. (sys_on_time): Ditto. (sys_on_signal): Ditto.

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