Introduce routines in each game module to encode a set of game

Introduce routines in each game module to encode a set of game
parameters as a string, and decode it again. This is used in
midend.c to prepend the game parameters to the game seed, so that
copying out of the Specific box is sufficient to completely specify
the game you were playing.
Throughout development of these games I have referred to `seed'
internally, and `game ID' externally. Now there's a measurable
difference between them! :-)

[originally from svn r4231]