Move notify_* and signal handling functions to their own headers.

This lets several files stop depending on externs.h at all. Yay!
Final projects on this list at the moment are merging ansi.h and
markup.h and figuring out a better place for mush_panic*.

git-svn-id: https://pennmush.googlecode.com/svn/trunk@1790 ba372814-4f39-11de-9ad6-1127a62b9fcd