Fixed|libdoom: Cheats not working in multiplayer

There were numerous logic issues in the cheat code handling which
interpreted cheats entered via the console differently to those
entered using keyboard input event sequences.

All cheats are now actioned by console commands which the original
games' keyboard input event sequences are mapped to. It no longer
matters how a cheat is entered.

All P_Give* functions have been reworked and cleaned up so as to
standardize on the internal APIs.

Todo: Do the same for libheretic and libhexen...