Improve virginity test for engine-defined pieces

Improve virginity test for engine-defined pieces

Two squares in the board are now reserved for flags that indicate
whether back-rank pieces have been touched. This allows MovesFromString()
to accurately test virginity of these pieces, rather than having to assume
it when the piece matches that in the opening position. For other ranks
the latter test is still used, as these are normally (irreversible) Pawns,
which cannot return there (and in drop games, when they would, would again
be considered virgin enough for the purpose of double-pushing!).