(top-level): Require mm-util for mm-make-temp-file.

(top-level): Require mm-util for mm-make-temp-file.
(mml2015-use): Prefer PGG if installed.
(mml2015-function-alist): Add PGG wrappers.
(mml2015-gpg-extract-signature-details): Check mml2015-use too.
(mml2015-gpg-extract-signature-details): PGG strips "gpg: "
prefix, make regexp optionally skip it.
(mml2015-pgg-decrypt, mml2015-pgg-clear-decrypt)
(mml2015-pgg-verify, mml2015-pgg-clear-verify, mml2015-pgg-sign)
(mml2015-pgg-encrypt): New functions.