mint-arena: Fix Team Arena post game not getting mouse move events

Add system call to call server shutdown _now_, instead of using
sv_killserver to shutdown server _later_.

Now Mouse State is set _after_ server is shutdown (before it was just after setting sv_killserver),
so cgame can receive mouse events.

Somewhat clean up cgame variables when killing server from UI.
Connecting to or starting a server will cause cgame restart though.