Implement engine-defined pieces

Implement engine-defined pieces

When legality testing is off, XBoard now listens to 'piece' commands
to redefine its move generator. After such a piece command is received,
target-square marking also can be used when legality testing is off.
The engine keeps in charge of legality testing, though.
 Only a very basic version of Betza notation is currently understood for
the move description: mode modifiers have to appear in the order mcd,
and gpz modifiers are ignored. Multi-leg moves as in Betza 2.0 is not
implemented at all. A leading i modifier limits the moves to the first
two ranks, which is no good at all. (But XBoard does not keep track of
which pieces have moved, and this at least would work for Pawns in a
FIDE-like setup, in particular Berolina.)