Suppress testing for availability in bughouse drops

Suppress testing for availability in bughouse drops

Give up on testing legality of bughouse drop moves; this only causes problems,
because the move list sent by the ICS is not accompanied by holdings updates.
So XBoard parses it thinking the holdings are empty, making every drop move
an ImpossibleMove. Bughouse can only be played on ICS, and we might as well
trust the ICS that it will only pass us legal moves. A user should not be
able to enter illegal drops; unavailable pieces cannot be grabbed from the
holdings, and typing is diverted to the ICS directly (which would reject the
illegal drop without XBoard even knowing about it).
  Also improve switching to bh/zh, based on drop move in initial board.