Switch to extinction-Chess win condition

Switch to extinction-Chess win condition

Normal Chess is of course a form of extinction Chess where you have to
exterminate the King piece type, since you only have a single one.
Royalty is still indicated by negative piece value, bt a win now only
occurs if you capture the last royal piece of its kind, rather than the
first.

The 'duple check' of Spartan Chess is treated as an exception, however:
when there is choice between royal pieces to capture, this is sufficient
for a win.