- Add a new flag, -xt, which enables inclusion of TSOs in a heap profile.

- Include large objects in heap profiles (except TSOs unless the -xt flag
  is given).

- In order to make this work, I had to set the bd->free field of the
  block descriptor for a large object to the correct value.  Previously,
  it pointed to the start of the block (i.e. the same as bd->start).
  I hope this doesn't have any other consequences; it looks more
  correct this way in any case.