spearmint: Fix systeminfo after fs_game not being set on client

When fs_game was found the variable s was set to value of sv_gameTitle.
This caused a cvar with name from game title to be created and stopped
parsing the system info string. sv_pure and sv_cheats are after fs_game,
so they were not being set. Oops.
spearmint: Only check CVAR_NONEXISTENT in Cvar_Server_Set once
spearmint: Limit what Cvar_VM_Set can force change

Don't let VMs instantly change latched engine cvars. It can cause crashes
or other unhandled behavior. trap_Cvar_Set( "sv_maxclients", "100" ) use to
crash if server was running.

Don't let VMs change cheat cvar values when cheats are disabled. This can be
exploited using custom Team Arena menus or a modified CGame VM.

Don't let CGame change system info cvars when connected to remote server.
This could be used for enabling sv_cheats. Exploitable using the same
methods as setting cheat cvar values.

Only check for CVAR_NONEXISTENT once in Cvar_VM_Set.