loader: Bump all versions to 3.0

Each incompatible change we make, we bump the major version. We've not
done the bump in a while, so sync everybody to 3.0. Anything older than
3.0 will be given a warning that their boot loader is too old. We check
only the major version, though, so minor versions can still be bumped
for individual loaders (though I honestly doubt we'll ever need to do
that again).

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D45888