Refactor mml-smime.el, mml1991.el, mml2015.el

Refactor mml-smime.el, mml1991.el, mml2015.el

(Maybe this is the last merge from Gnus git to Emacs git)

Cf. discussion on ding mailing list, messages in
<http://thread.gmane.org/gmane.emacs.gnus.general/86228>.
Common code from the three files mml-smime.el, mml1991.el, and
mml2015.el is moved to mml-sec.el.  Auxiliary functions are added
to gnus-util.el.

The code is supported by test cases with necessary test keys.

Documentation in message.texi is updated.

* doc/misc/message.texi (Security, Using S/MIME):
Update for refactoring mml-smime.el, mml1991.el, mml2015.el.
(Using OpenPGP): Rename from "Using PGP/MIME"; update contents.
(Passphrase caching, Encrypt-to-self, Bcc Warning): New sections.

* lisp/gnus/gnus-util.el (gnus-test-list, gnus-subsetp, gnus-setdiff):
New functions.

* lisp/gnus/mml-sec.el: Require gnus-util and epg.
(epa--select-keys): Autoload.
(mml-signencrypt-style-alist, mml-secure-cache-passphrase): Doc fix.
(mml-secure-openpgp-signers): New user option;
make mml1991-signers and mml2015-signers obsolete aliases to it.
(mml-secure-smime-signers): New user option;
make mml-smime-signers an obsolete alias to it.
(mml-secure-openpgp-encrypt-to-self): New user option;
make mml1991-encrypt-to-self and mml2015-encrypt-to-self obsolete
aliases to it.
(mml-secure-smime-encrypt-to-self): New user option;
make mml-smime-encrypt-to-self an obsolete alias to it.
(mml-secure-openpgp-sign-with-sender): New user option;
make mml2015-sign-with-sender an obsolete alias to it.
(mml-secure-smime-sign-with-sender): New user option;
make mml-smime-sign-with-sender an obsolete alias to it.
(mml-secure-openpgp-always-trust): New user option;
make mml2015-always-trust an obsolete alias to it.
(mml-secure-fail-when-key-problem, mml-secure-key-preferences):
New user options.
(mml-secure-cust-usage-lookup, mml-secure-cust-fpr-lookup)
(mml-secure-cust-record-keys, mml-secure-cust-remove-keys)
(mml-secure-add-secret-key-id, mml-secure-clear-secret-key-id-list)
(mml-secure-cache-passphrase-p, mml-secure-cache-expiry-interval)
(mml-secure-passphrase-callback, mml-secure-check-user-id)
(mml-secure-secret-key-exists-p, mml-secure-check-sub-key)
(mml-secure-find-usable-keys, mml-secure-select-preferred-keys)
(mml-secure-fingerprint, mml-secure-filter-keys)
(mml-secure-normalize-cust-name, mml-secure-select-keys)
(mml-secure-select-keys-1, mml-secure-signer-names, mml-secure-signers)
(mml-secure-self-recipients, mml-secure-recipients)
(mml-secure-epg-encrypt, mml-secure-epg-sign): New functions.

* lisp/gnus/mml-smime.el: Require epg;
refactor declaration and autoloading of epg functions.
(mml-smime-use): Doc fix.
(mml-smime-cache-passphrase, mml-smime-passphrase-cache-expiry):
Obsolete.
(mml-smime-get-dns-cert, mml-smime-get-ldap-cert):
Use format instead of gnus-format-message.
(mml-smime-epg-secret-key-id-list): Remove variable.
(mml-smime-epg-passphrase-callback, mml-smime-epg-find-usable-key)
(mml-smime-epg-find-usable-secret-key): Remove functions.
(mml-smime-epg-sign, mml-smime-epg-encrypt): Refactor.

* lisp/gnus/mml1991.el (mml1991-cache-passphrase)
(mml1991-passphrase-cache-expiry): Obsolete.
(mml1991-epg-secret-key-id-list): Remove variable.
(mml1991-epg-passphrase-callback, mml1991-epg-find-usable-key)
(mml1991-epg-find-usable-secret-key): Remove functions.
(mml1991-epg-sign, mml1991-epg-encrypt): Refactor.

* lisp/gnus/mml2015.el (mml2015-cache-passphrase)
(mml2015-passphrase-cache-expiry): Obsolete.
(mml2015-epg-secret-key-id-list): Remove variable.
(mml2015-epg-passphrase-callback, mml2015-epg-check-user-id)
(mml2015-epg-check-sub-key, mml2015-epg-find-usable-key)
(mml2015-epg-find-usable-secret-key): Remove functions.
(mml2015-epg-decrypt, mml2015-epg-clear-decrypt, mml2015-epg-sign)
(mml2015-epg-encrypt): Refactor.