util_run_program: restore signal mask before executing event RUN commands

External programs triggered by events (via RUN=) will inherit udev's
signal mask, which is set to block all but SIGALRM. For most utilities,
this is OK, but if we start daemons from RUN=, we run into trouble
(especially as SIGCHLD is blocked).

This change saves the original sigmask when udev starts, and restores it
just before we exec() the external command.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>