gpg: Use just the addrspec from the Signer's UID.

* g10/parse-packet.c (parse_signature): Take only the addrspec from a
Signer's UID subpacket.

This is to address a problem in the currentr OpenKeychain which put
the entire UID into the subpacket. For example our Tofu code can only
use the addrspec and not the entire UID.

• Reported-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>
• Signed-off-by: Werner Koch <wk@gnupg.org>