Enhance icon.pl to be a fully general Windows icon builder: it now

Enhance icon.pl to be a fully general Windows icon builder: it now
supports monochrome icons, can deal with any size of image you're
mad enough to put in there, and will construct icons with whatever
combination of sizes and resolutions you feel like specifying. This
has involved a change in the command-line syntax, hence the
adjustment to Makefile.

(I don't imagine that the changes described here will be critical to
Puzzles any time soon, but I might reuse this script elsewhere and
then I won't want it to have arbitrary limitations.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@7031 cda61777-01e9-0310-a592-d414129be87e