Only perform e.p. capture if there are rights

Only perform e.p. capture if there are rights

The heuristic for e.p. capture was such that any non-forward move with
a Pawn to an empty square would delete the piece behind its new location
(Xiangqi exempted). This is now limited to cases where EP_STATUS indicates
that the deleted piece was a Pawn that performed a double-push on the
previous move. (Or if there is unknown EP_STATUS.)
 This makes XBoard more generally suitable for use with variants that have
non-conventional Pawn moves (with legality testing off). To guarantee this
will not have any unexpected effects in normal Chess, this patch will only
be active if legality testing is off.