Fix drop moves on boards with more than 10 ranks

Fix drop moves on boards with more than 10 ranks

The internal encoding for rank 16 is '@', and thus ambiguous with drop
moves. They were always printed as 16, breaking drop games on large
boards. The ambiguity is now solved (for boards with more than 16 ranks)
by checking if the preceeding letter is upper case, in which case it
cannot be a fileindicator, but must be a piece, and thus a drop.