Move extlib.h to include/rackmate.h
Rename RACKIT_GUI to RACKMATE_GUI
Make all warnings errors for debug builds
~/Library/Cache/Rackmate -> ~/.../co.rackit.mate

Changed because this is more typical and various NSFoo will assume these directories, so we'll get two.
Mac updater logic
Fix crash in shut-down
Prevent sockets getting into forks