Implement the remaining modes of reasoning in nsolve, and thus

Implement the remaining modes of reasoning in nsolve, and thus
enable configurable puzzle difficulty. I'm only generating grids up
to Times level (complicated non-recursive analysis but guessing
never required); I wouldn't object to providing a Telegraph
difficulty level (guessing required) but it turns out to be very
hard indeed to generate at random. I might still add it later
(probably under the name `Unreasonable' :-) if I can think of an
efficient way to find them.

[originally from svn r5682]