new cvar: quit_when_empty - exits the server once it is empty (apart from bots)

new cvar: quit_when_empty - exits the server once it is empty (apart from bots)
at the end of a game. Useful for server admins who use a service manager (like
svsvinit, daemontools, svc or, if nothing else is there, cron or a shell script
with an endless loop) that automatically restarts the server when needed, that
way, they can schedule an engine update to when the server is empty.

git-svn-id: svn://svn.icculus.org/nexuiz/trunk@2055 f962a42d-fe04-0410-a3ab-8c8b0445ebaa