Giant const patch of doom: add a 'const' to every parameter in every

Giant const patch of doom: add a 'const' to every parameter in every
puzzle backend function which ought to have it, and propagate those
consts through to per-puzzle subroutines as needed.

I've recently had to do that to a few specific parameters which were
being misused by particular puzzles (r9657, r9830), which suggests
that it's probably a good idea to do the whole lot pre-emptively
before the next such problem shows up.

[originally from svn r9832]
[r9657 == 3b250baa02a7332510685948bf17576c397b8ceb]
[r9830 == 0b93de904a98f119b1a95d3a53029f1ed4bfb9b3]