Add mml-smime-encrypt-to-self.

* mml-smime.el (mml-smime-encrypt-to-self): New user option analogous
to mml2015-encrypt-to-self.
(mml-smime-epg-encrypt): Respect mml-smime-encrypt-to-self.
color: fix color-rgb-to-hsv computing

Signed-off-by: Julien Danjou <julien@danjou.info>
gnus-art.el, gnus-async.el: Use point-min-marker and point-max-marker
gnus-start.el, gnus-sum.el: Fix ambiguous doc string cross-references