Allow signal callback deletion while signalling

I encountered a situation where I wanted to delete a callback for a
signal while inside of that signal.  However it would hard lock, and
even after that, it would mess up the loop for the callback list.

So, change the mutex of the individual signals to a recursive-style
mutex, and then if a callback of a signal is deleted while currently in
that signal, just mark it for deletion, which will happen after the
signal is complete.