VMs could set fs_game to '..' at anytime to access files outside of home
and base path. fs_game sent by server to clients could also be '..' to
access files outside of home and base path.
'..' was not caught by FS_CheckDirTraversal() as it expects filenames
not a single directory.
I've made fs_game be latched to prevent VMs from changing it with no
good way to validate it before it's used. com_basegame and fs_basegame
are now latched as well.
Additionally, it's now possible to change com_basegame while the engine
is running. game_restart or vid_restart will make it take affect.
com_homepath is now CVAR_PROTECTED to prevent VMs from changing it
to a directory traversal.
This requires my two previous commits for preventing VMs from changing
engine latch cvars and only Cvar_Get fs_game in FS_Startup (so CVAR_INIT
isn't added in serveral other places).
Reported by Noah Metzger (Chomenor).
3638f69dff