Allow setting up position from scratch

Allow setting up position from scratch

The position string in Bonanza's native 'new' command can now have a new
format, starting with 'SU' in stead of 'PI', which adds the mentioned pieces
to an empty board, rather than deleting them from the standard initial
position. Holdings pieces are encoded by '00' for square coordinates.
A '+' or '-' embedded in the string can be used to switch between Sente
and Gote. (By default it assumes Sente, so all Gote pieces could be mentioned
at the end after a single '-'.)