Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

shr.el: Use defface to create shr-tag-h[1-6] faces to fontify h[1-6] tags.
mml-smime.el (mml-smime-use): Make it a defcustom and default to 'epg if EPG is loaded.
message.texi (IDNA): Explain what it is.
gnus.texi (The Empty Backend): Document nnnil (bug #7653).
gnus-agent.el (gnus-agent-prompt-send-queue): Whitespace fix.
shr.el (shr-expand-newlines): Proof of concept implemantation of boxy backgrounds.
 (shr-expand-newlines): Switch to using overlays to enable kill'n'yank in a more sensible manner.