Aha! It turns out, after a bit of failure-mode profiling, that when

the Mines unique grid generator fails at high mine densities it is
_almost always_ for the same reason, and it also turns out that this
reason is one which can be addressed. So here's an enhancement to
mineperturb() which enables Mines to generate a grid at (as far as I
can tell) any mine density you like, up to and including w*h-9
mines. At densities of 1 in 2 or thereabouts the grids start to look
rather strange, but it can at least generate them without hanging.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5885 cda61777-01e9-0310-a592-d414129be87e