prepare for public release, add licence

[jclulow]

This change seeks to fix #115 (and, incidentally, surpasses #111 and perhaps #117 by virtue of updating dependencies), as well as generally straighten things up ready for public release.

I have split the change into several commits to make it easier to review, but have tested it all together so I'll squash and merge when ready.

In particular I have tested:

• cloning the repository as an otherwise unprivileged user (I took my keys away before gmake setup), building illumos from stlouis, installing the build locally in a fresh VM, etc
• making my keys available again, I was able to clone the missing private stuff needed to build a Gimlet image which I have booted on sn66 (paris) to confirm things are not broken there

Note that I have made everything else for public release open already; i.e.,

• oxidecomputer/helios-engvm
• oxidecomputer/helios-omnios-build
• oxidecomputer/helios-omnios-extra
• oxidecomputer/boot-image-tools
• oxidecomputer/garbage-compactor

This is the last one!

[citrus-it]

This looks great, thanks. I especially like the table of incorporations in the main README, that's a useful reference for all of us.

[bcantrill]

Looks great -- thank you!