Fix deselection of piece

Fix deselection of piece

The patch to allow entering of friendly capture (intended as a kludge
for entering non-standard castling) had broken the ability to change
the selected piece by clicking another piece, as this was now always
interpreted as a friendly capture (which was then rejected as illegal).
By testing marker[][] in stead of legal[][] this can be avoided; legal[][]
was not a good measure, because in absence of a highlight command it
is completely filled with 1, to make everything legal. No friendly squares
will ever get marked unless a highlight command does it, though.