unify formatting of fingerprints

This change introduces a common function to turn
a fingerprint into a human readable format.
display key fingerprints if offered by keyserver

This change tries to make use of complete key fingerprints if those are
offered by the consulted keyserver. If a keyserver returns the entire
fingerprint as a user id, it will be shown to the user searching the
database.
request and process fingerprint from keyservers

This change also introduces an optional "fpr" line to keyserver answers,
which is expected to contain the fingerprint of the returned key. This
can be used to distinguish keys from each other, even if a collision of
the (short) key ids has occured and the key format does not allow a
transformation between key id and fingerprint.