Until now, fatal_signal_fork() has simply disabled all the fatal signal
callback hooks. This worked fine, because a daemon process forked only
once and the parent didn't do much before it exited.
But upcoming commits will introduce a --monitor option, which requires
processes to fork multiple times. Sometimes the parent process will fork,
then run for a while, then fork again. It's not good to disable the
hooks in the child process in such a case, because that prevents e.g.
pidfiles from being removed at the child's exit.
So this commit changes the semantics of fatal_signal_fork() to just
clearing out hooks. After hooks are cleared, new hooks can be added and
will be executed on process termination in the usual way.
This commit also introduces a cancellation callback function so that a
canceled hook can free resources.
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.
