make fork()ing GMainContext explode sooner
Submitted by Allison (desrt)
trying to have a GMainContext across a fork() is a recipe for intense amounts of pain. first: the worker thread doesn't follow you. second: you're now sharing an eventfd with your parent.
sometimes you can kind of get away with it and end up with weird bugs that are difficult to track down. we should make a point of asserting quite rapidly if we detect this situation.