sysupgrade: add -R #.# to try to use a specific release version rather

sysupgrade: add -R #.# to try to use a specific release version rather
than the immediate +0.1. print an https://ftp.openbsd.org/... URL where
the new signify pubkey can be found if not present.

no guarantees: we only test +0.1, but jumping further does work quite
often (and if tight on disk, can work better than multiple steps) -
this avoids editing the script if you're going to do it anyway.

"Only upgrades from one version to the next are tested. Skipping
versions may work. Downgrading is unlikely to work."

discussed with deraadt chris florian, ok deraadt