Allow setting of piece nicknames from pieceToChar string

Allow setting of piece nicknames from pieceToChar string

A piece IDs in the pieceToChar string can now be suffixed by "=L",
with L an arbitrary single letter, to define a nickname for the piece.
Such a nickname would make it possible to specify a complex ID like +I'
as promotion choice, and by indicating this in the pieceToChar string
the engine can configure that through the 'setup' command. This facility
is needed to force promotion after capture of a contageous piece in
variants with Shogi promotion, such as Maka Dai Dai Shogi.