change references to GNUS to gnus

[Dan Jacobson]

the Help for mail-user-agent says:

  Valid values include:

    sendmail-user-agent -- use the default Emacs Mail package
    mh-e-user-agent     -- use the Emacs interface to the MH mail system
    message-user-agent  -- use the GNUS mail sending package

But GNUS refers to old gnus, so decapitalize it, and look for other
occurrences of GNUS thruout emacs and its docs to see if something
needs updating.  Of course some items really are specific to old GNUS,
so append a note to them in that case.
-- 
http://www.geocities.com/jidanni Tel886-4-25854780 e-mail:restore .com.