It's a 32-bit-access-only heap. On the Ultimate Doom WAD the amount of
data moved is about 17 kiB, which is not a lot, but arrays with 4-byte
elements are not easy to find in Doom.
lumpinfo is now allocated in Z_Malloc because it's needed for some
larger WADs.
More heap is needed to compensate and to support larger WADs fully, so
the unused part of the user stack is added as a second zone.
This makes at least the start of the DOOM Ultimate WAD playable.