Detect if a message can be encrypted and add an MML tag

Detect if a message can be encrypted and add an MML tag

* lisp/gnus/message.el (message-all-recipients): Return a list of
pairs, one for each recipient in To, Cc, Bcc.
(message-all-epg-keys-available-p): Check that there is a public key
in epg for each recipient of the current message.
(message-sign-encrypt-if-all-keys-available): Add MML tag to sign and
encrypt current message if there is a public key for every recipient
in current message.

* test/lisp/gnus/message-tests.el (message-all-recipients): Test for
message-all-recipients.