Allow for an arbitrary number of splits of the system memory map via
INT 15,e820.

Features of the new map-mangling algorithm include:

  Supports random access to e820 map entries.

  Requires only sequential access support from the underlying e820
  map, even if our caller uses random access.

  Empty regions will always be stripped.

  Always terminates with %ebx=0, even if the underlying map terminates
  with CF=1.

  Allows for an arbitrary number of hidden regions, with underlying
  regions split into as many subregions as necessary.

Total size increase to achieve this is 193 bytes.

wasn't meant for my local "master" branch ;)
This reverts commit 373022108b.

below _text.  This should help with the gPXE-on-gPXE-via-PXE case.

reflect the fact that they allocate and deallocate user memory (i.e.
things reached through a userptr_t).

alignment.