Fix signal safety deadlock

[LiterallyVoid]

Fixes #4047.

This creates a separate thread to watch for signals. I used a pipe to send signals to the signal thread, as pipes are one of the few signal-safe messaging options.

Because GTK isn't thread safe, I couldn't handle signals in the signal thread either, so I used a SafeSignal which is thread-safe (but probably not signal-safe!) so that signals were handled on the main thread.

I also moved the SIGCHLD handling code into the new thread. I'm unsure about this change, as it will longer mask SIGCHLD in the main thread. From a cursory test it seems to work and children are still being reaped correctly, but I separated this change into its own commit so it can be reverted separately.

(Incidentally, all of reload's accesses were on the same thread, so I changed it from a global to a local.)