Gary Wong observes that solo's decode_params() is overenthusiastic

Gary Wong observes that solo's decode_params() is overenthusiastic
about eating the letter `d' (for `diagonal') when it appears in a
symmtery description: it should only be used after `m', because
mirror symmetry is the only type that can be diagonal. This was
causing parsing of the parameter description `3x3adu' to produce the
wrong answer: the d would be swallowed, then the u ignored for being
incomprehensible, and you'd get default Trivial difficulty.

[originally from svn r7386]