[PATCH gpg] allow import without user ids

[Vincent Breitmoser]

This patch allows import of keys that have no user ids, if they are already
known. This allows update of subkeys, revocation certificates, etc from sources
that don't deal with user ids. See also https://dev.gnupg.org/T4393

Second commit is simple bugfix to the keyserver screener, which memcmp'd 32
bytes rather than the actual length of the fpr to compare.

Signed-Off-By: Vincent Breitmoser <look at my.amazin.horse>