Improve SAN of Pawn moves and allow Betza e.p. definition

Improve SAN of Pawn moves and allow Betza e.p. definition

The move descriptions in 'piece' commands can now use 'i' and 'n'
modifiers to define a blockable double push. ('n' only work for moves
of stride 2!) The test for initial moves now is based on comparison with
the opening position, rather than on a way-to-generous heuristic. (This
still does not prove in generak the piece did not move, though, but it
does for Pawns.) A new 'e' modifier stands for e.p. capture; to implement
this the rank and file of the e.p. square are now stored in the board.
(EP_STATUS was holding the file of the moved Pawn, which in Berolina Chess
is not the file of the e.p. square!)
 The generation of SAN moves for Pawns can now reliably distinguish between
captures and non-captures even in Berolina, when the move generator informs
it whether the move was e.p. capture.