Misc gamedir-related fixes

Misc gamedir-related fixes

Fixes two cvars being writable that shouldn't be.

Fixes MVM getgamedirinfo() builtin returning stale data (wasn't updated
by fs_rescan).

Fixes `startdemos` not working when changing to a mod that uses it,
after previously having had a map loaded.

Fixes `menu_main` being unavailable to quake.rc when changing to a mod
that uses it instead of `startdemos`.

Fixes `menu_restart` not performing the full MR_Init() (updates video
modes etc).

Fixes a race where it was possible to segfault by handling keyboard
events meant for MENU QC after MR_Shutdown() but before MR_Init() (by
calling directly instead of adding `menu_restart` to the cbuf).

Fixes an inconsistent extern declaration found by ubsan.

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