Fix reported DTZ for mate-in-1-ply positions

Fix reported DTZ for mate-in-1-ply positions

This corrects a bug in Tablebases::probe_dtz() which sometimes causes
a higher DTZ value to be returned for the position one ply before mate
than for the position two plies before mate.

The problem was reported by Kolja Kühn here:
http://talkchess.com/forum/viewtopic.php?p=757497#757497

It is explained here:
http://talkchess.com/forum/viewtopic.php?p=757506#757506

I have also adjusted some comments to make clear that probe_dtz()
returns -1 for a mate position.

Closes https://github.com/official-stockfish/Stockfish/pull/1546

No functional change