Add 'const' to the game_params arguments in validate_desc and

Add 'const' to the game_params arguments in validate_desc and
new_desc. Oddities in the 'make test' output brought to my attention
that a few puzzles have been modifying their input game_params for
various reasons; they shouldn't do that, because that's the
game_params held permanently by the midend and it will affect
subsequent game generations if they modify it. So now those arguments
are const, and all the games which previously modified their
game_params now take a copy and modify that instead.

[originally from svn r9830]