If you pass --key to download-build and signed packages aren't
available, Koji will skip the unsigned package, or error out.
This adds a modified behavior controlled by the new
--fallback-unsigned arg. If this is passed with --key, unsigned
copies will be downloaded for packages for which no signed copy
can be found.

This is primarily intended to work with a proposed Bodhi feature:
https://github.com/fedora-infra/bodhi/pull/5859 . That would
make Bodhi's bodhi updates download command automatically try
to download signed copies, but I think it would be best if it
falls back to getting unsigned copies if that doesn't work. Just
failing out entirely seems wrong for that case. Implementing the
fallback in Bodhi itself is more awkward and messy than adding it
in Koji, and it may be useful for others in Koji I guess.

Note there are two distinct 'no signed copies' cases. In the
simple one, queryRPMSigs tells us Koji has no record of the
package ever being signed with the key in question. In this case
we don't bother trying to download a signed copy. In the other
case, queryRPMSigs tells us the package has been signed with
the key, but it turns out that signed copy has been garbage-
collected and we can no longer download it. In this case we have
to catch the failure on the download attempt and retry the
download with sigkey set to None.

Signed-off-by: Adam Williamson awilliam@redhat.com