digging conjoined pits (trunk only)

digging conjoined pits (trunk only)

This one turned out to be more effort than I had
originally anticipated.

We had a bug report requesting that zapping a wand of digging
laterally while in a pit should dig beside you. That seemed
like a reasonable enough request, but this ended up with
the following results:
- needed to check where this should not be permitted, or at
  least where there should be special-case code because there is
  something such as furniture on the surface above the dig
  point.
- now tracks conjoined pits through new fields in the trap
  structure, hence the pathlevel increment. The array of
  8 boolean values represents each of the 8 directions
  around a pit.
- Previously, pits could be adjacent to each other as two
  individual pits, in which case moving between them
  results in a fall as you went into the next pit. That
  behavior is preserved.
- Pits created either by zapping a wand of digging
  laterally while in a pit, or by "clearing debris"
  between two adjacent pits via a pick-axe, sets the
  conjoined fields for those two pits. You cannot
  create a brand new adjacent pit via pick-axe, only
  with the wand.
- The hero can pass between conjoined pits without
  falling.
- dighole() was hijacked for adjacent pit digging,
  so the ability to pass coordinates to it and
  its downstream functions was added (dig_up_grave()
  for example). dighole() does pretty much everything
  appropriately for this adjacent digging, more so
  than calling digactualhole() directly.
- moving into a conjoined pit that has spikes still
  does damage, but less so that "falling" into the
  spiked pit, and you "step on" the spikes rather
  than falling into them.
- Not done: should pits with the conjoined fields
  set be referred to as 'trenches' rather than pits
  in messages and identifications?