Always accept piece commands in variant great

Great Shatranj belongs to the variants XBoard does not know the rules of,
and should thus always accept engine piece commands.
Update Game List after tag selection changed

When the Game List Options dialog changes the tags to be displayed in
the Game List lines, we now automatically redo the entire Game List.
Fix some uninitialized variable bugs

The writing of Seirawan castling rights in FEN was still dependent on a
now unused variable, and encountering a VariantMen tag in a PGN file
could have created the misconception the memory was full.
Preserve parent variant for PGN of engine-defined game

Rather than decoding an unknown variant name, (which will result in
'normal'), we keep the currently set (parent) variant when an
engine-defined variant is currently set that matches the name in
the PGN variant tag of the loaded game.
Fix loading of engine-defined PGN games

Somehow there could be disagreement over what the official opening
position of an engine-defined variant was during loading of the game.
It then refused Betza-defined castling, which tests the corner pieces
based on this initial position. We now assume the FEN tag, which such
a PGN game will always contain, holds the official opening position,
so that castling will always be assumed possible (if there is a corner
piece).