Mark castling Rook as non-virgin

Mark castling Rook as non-virgin

The Rook was left virgin when castling. This could cause erroneous gating
in variant seirawan. Now the original piece is remembered in a variable rk,
from which it can be restored, while the on-board (moved) Rook wil get the
virginity bit set. Separate storage was preferred over clearing the bit
on take-back, because it allows the same code to also handle other removal
of pieces a a side effect of a move, by setting G to the piece to be removed.