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...
Fixed|libheretic: Cheats not working in multiplayer

See: b9fdf4fcc9086627c0b9c30d3a3d52ebc471c3ba

Todo: The cheats 'gimme', 'iddqd' and 'idkfa' require various new
features and/or functionality. I'll return to these once libhexen
has been updated...
Fix typo
Fixed|libheretic: Cheats not working in multiplayer

See: b9fdf4fcc9086627c0b9c30d3a3d52ebc471c3ba
libheretic: Further multiplayer cheat code entry fixes
Cleanup
libdoom: Handle the 'kill' command as a cheat in multiplayer