Rather than running signal hooks directly from the actual signal
handler, simply record the fact that the signal occured and run
the hook next time around the poll loop. This allows significantly
more freedom as to what can actually be done in the signal hooks.
Some systems complain when certain functions' return values are not
checked. This commit fixes those warnings.
Creating ignore() function suggested by Ben Pfaff.
In an upcoming commit, ovs-brcompatd will need to create a subprocess and
capture both its stdout and stderr separately, which one cannot do with
simple interfaces such as popen(). This function provides that ability.
To prevent fatal signals in a child process from causing the parent
process's pidfile, etc. to be deleted, we need to block fatal signals
around fork and call fatal_signal_fork() in the child process.
This problem was noticed through code inspection; it has not been observed
in practice.
