Add some awareness of short file names

- Added I_GetLongPathName(). It wraps the Win32 API's GetLongPathName().
  DArgs::CollectFiles() now calls this for every argument it processes, so
  any arguments passed using short file names will be converted to long
  file names. This is mainly of interest so that savegames will never
  record the short file name, which can change based on what else is in
  the directory.
Fixed: Make A_SetTics work with weapons.

- When A_SetTics is called from a weapon, we need to set the tics for the
  psprite instead of the actor itself.
Fixed: CheckPlayerCamera not sync safe

-  If a player is spying through another player, CheckPlayerCamera will
   return the TID of the player you are "spying", but as coopspy isn't a
   net command, this wont be reflected by all nodes. So to fix this,
   CheckPlayerCamera now returns -1 if a player's camera is that of any
   player at all. (thanks edward850)
Reset FirstFreeEntry in ACSStringPool::ReadStrings()

- Fixed: When an ACS string pool was read from a savegame, FirstFreeEntry
  would not be updatedt, except by the Clear() function. This left FirstFreeEntry
  at 0, which meant the next string added to the pool would always go in
  slot 0, whether it was free or not.