emacs: Prefer email address over User ID when showing valid signature

emacs: Prefer email address over User ID when showing valid signature

Most concrete verification steps are likely only taken on the e-mail
address in the first place, and e-mail addresses render more
intelligibly than arbitrary User IDs in the first place.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Amended-by: db, apply dme restructuring suggestions.