…ed keyring

There may be a better way to display this information. This is only a first cut.
Most signatures are likely to be unknown. Label these as simply "Signature".
In the case that a signature is already in the trusted keyring, label it
"Trusted signature".

There are at least two pieces of information that we need
to display for each signature: the "familiarity" of the
signature (unknown, known, or trusted) and the key ID of
the signature.

This data might be easier to display to the user in a
tabular format, as in this commit.