Add a new dvonn_board_cell_owned_by function

The compiler was warning that we were comparing enums of two different
types. That's true—we are, but that's because the values for
dvonn_cell_t are declared by reference to dvonn_player_t values.

Anyway, rather than cluttering up the code with lots of casts to
silence this warning, here we add a new function that takes care of
that cast, leading to more readable code.