Astrometry.net 0.85: fix a licensing issue

Primarily, this is to fix a mistake in our licensing: we accidentally had a bit of GPL v2-only code (from the Linux kernel) included; this has been replaced by a BSD-sourced version.

All other changes are minor.

Note that the main development branch name on Github is now "main". Your git client will tell you how to adapt to this change the next time you 'git pull'. It should mostly be transparent.