Implement move borrowing in XBetza

Implement move borrowing in XBetza

The XBetza 'x' modifier can now be used in final legs to indicate move
borrowing from a friendly target. This only works when the target's move
had an XBetza definition (i.e. was defined by a 'piece' command).
Although borrowing from equal type is suppressed, there is a danger
of infinite recursion when two different move borrowers occur in one game.
XBoard tries to suppress recursive move borrowing in general, by not
executing borrowing specs in a borrowed move, but a preceding leg can
hide this.