Fixed a major security hole in admin commands

Renamed admin command public function name prefix to "acmd_" rather than
"cmd_" (which is the same as regular commands, so admin commands could
be accessed simply by typing an underscore and the admin level)

Fixed a bug that allowed players to abuse the anti-car-surf which
boosted them upwards by a metre, this code now moves them back in the
opposite direction of the vehicle's velocity.