Emacs 'movemail' is now a configure-time option

Emacs 'movemail' is now a configure-time option

The new configure option --with-mailutils lets the builder say
that Emacs should assume that GNU Mailutils is installed, instead
of continuing to build and install its own limited and insecure
substitute for 'movemail'.
* INSTALL, etc/NEWS, etc/PROBLEMS: Mention --with-mailutils.
* configure.ac: Add --with-mailutils option.
(with_mailutils): New variable.
Do not bother configuring 'movemail' when not building it.
Warn about issues relating to --with-mailutils.
* doc/emacs/rmail.texi (Movemail): Mention --with-mailutils.
(Movemail, Remote Mailboxes): Document port numbers in
POP and IMAP URLs.
* lib-src/Makefile.in (with_mailutils): New macro.
(UTILITIES): Use it.