Addfile command verify fix.
Fixes bug reported at https://mb.srb2.org/showthread.php?p=802000. Where the game would attempt to send XD_REQADDFILE to itself if the adminplayer is the serverplayer, causing the performance to drop as a result. Also disabled the ability to use login
and verify
in splitscreen or single player. As I see no reason for someone to use those commands.