New configuration option in Unruly, to enable a mode in which no two

New configuration option in Unruly, to enable a mode in which no two
rows are permitted to have exactly the same pattern, and likewise
columns. (A row and a column can match, though.) This rule is
sometimes used, but not always; http://www.binarypuzzle.com/ in
particular has it (as a user just pointed out).

Since the filled-grid-generation stage of Unruly is implemented by
repeatedly choosing a square and then calling the solver to fill in
all its consequences, simply adding a solver mode conditional on this
flag is sufficient to do the whole of the interesting part of the job.

[originally from svn r9837]