Sanity-checking patch from Phil Bordelon: since Solo can't cope with

more than 36 distinct symbols (it runs out of alphanumerics), check
this in validate_params. I hate to do this, since I like puzzle
sizes to at least be open-ended in _principle_, but in this case
there's a fundamental UI limitation which won't be fixed by getting
a faster CPU.

[originally from svn r6098]