Remove a lot of old and obsolete info.

Remove a lot of old and obsolete info.
(titlepage): Simplify.
(Emacs 19 MUAs, Emacs 18 MUAs, MH-E with any Emacsen)
(VM with any Emacsen, GNEWS with any Emacsen)
(Overloading for Non-conforming MUAs, Version 3 Changes)
(The Supercite Mailing List): Delete nodes.
(Introduction): Remove info about old packages.
(Getting Connected): Simplify.  Remove info about old packages.
(Citing Commands): Delete Emacs 19 info.
(Hints to MUA Authors): Simplify.
(Thanks and History): Merge in some info from the deleted node
"Version 3 Changes".