Extend legality testing to drop moves

Extend legality testing to drop moves

LegalityTest() can nowbe called with (internal-format) drop moves, and
delegates their testing to a new routine LegalDrop(). This routine
performs the 'no-pawn-on-back-rank' test that used to be in
UserMoveTest (which now calls LegalityTest also for drop moves).
The more complex Shogi case is also handled (except for Pawn mate
drops) in LegalDrop().