Retire move_to_san()

Now "Write Search Log" will pring moves in UCI format, consistent with all the rest. This functionality is
not aimed at end-users anyway. It's hardly useful at all, in fact. Also, pretty-printing SAN moves is
something that better belongs in the GUI than in the engine.

No functional change.
Fix perft 1

Compute correct number of moves for this corner case.

A smal bug crept in after recent perft rework.

No functional change.