* mml-smime.el (mml-smime-use): New variable; default to use openssl.

* mml-smime.el (mml-smime-use): New variable; default to use openssl.
(mml-smime-function-alist): New variable; add epg as the backend.
* mml-sec.el (mml-smime-sign): Don't require mml-smime, autoload
mml-smime- functions instead.
* mm-view.el: Require smime.