libcommon: Kludge around a compatibility issue with Hexen's MAPINFO

MAPINFO in the commercial versions of the Hexen IWADs contain a bunch
of broken definitions. As later definitions now replace earlier ones
(a change from ZDoom), these broken defs will now override the earlier
"good" defs.

For now we'll kludge around this issue by patching such defs with the
original/expected values.

Todo: Determine how ZDoom behaves and replicate that behavior.
Todo for later: Isolate this compatibility swamp by moving the MAPINFO
parser to load time translator plugin.