Tweak the icon script, and the generated icons, to more closely

Tweak the icon script, and the generated icons, to more closely
match the original icons. (Apparently I managed to introduce errors
while transcribing the originals for detailed analysis.)

While I'm at it, add the obviously useful `make install' target in
icons/Makefile, and fix the svn:ignore property on the icons
directory.

git-svn-id: svn://svn.tartarus.org/sgt/putty@7068 cda61777-01e9-0310-a592-d414129be87e