Bug - Disable Multi-tow

- Fixed exploit which allowed players to tow vehicles which were already
  being towed.
- Fixed exploit which allowed players to tow vehicles which were already
  towing another vehicle. This functionality can be turned back on via the
  MF_Tow_Multi_Towing config param being set to true (default value is
  false). Be warned, turning this on produces "interesting" results and
  probably only serves as a means for trolling.