SVVM: fully shut down when stopping a server to avoid segfaulting later

SVVM: fully shut down when stopping a server to avoid segfaulting later

Fixes segfaults when functions that check if the progs is loaded (to
avoid crashing) are called after stopping a server.
Example repro: load a map, disconnect, `set sv_cheats 0`.

Fixes wrong SV_Shutdown order: SV_DropClient must be done before
shutting down SVQC because it calls SVQC functions.

Deduplicates SVQC shutdown.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>