This removes the special handling of minor versions as major when the
actual major is zero, and adds the special case that upgrades from 0.x
to 1.x are considered minor. 0.x to 2.x or 1.x to 2.x etc are still
considered major.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4226
New signature is the HMAC of archive name (which includes the release
version and architecture) plus the contents of the binary. This is
expected in a new file "release.sig" which may be present in a
subdirectory. The new release tools put this in [.]metadata/release.sig.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3043
