Allow O1 as Betza castling descriptor

Allow O1 as Betza castling descriptor

The notation On is now also recognized as castling for n=1, but this
uses the same system as non-royal castling ('guarding'), which is
performed as hit-and-run capture of the Rook (or whatever you want to
castle with) while moving to the destination (one square away here).
This to avoid the ambiguity with normal King moves (which for the
guarding always existed).