Updated to work with movemail from GNU Mailutils

Updated to work with movemail from GNU Mailutils
(rmail-pop-password, rmail-pop-password-required): Moved to
rmail-obsolete group.
(rmail-set-pop-password): Renamed to rmail-set-remote-password.
All callers updated.
(rmail-get-pop-password): Renamed to rmail-get-remote-password.
Take an argument specifying whether it is POP or IMAP mailbox we
are using. All callers updated.
(rmail-pop-password-error): Renamed to rmail-remote-password-error.
Added mailutils-specific error message.
(rmail-movemail-search-path)
(rmail-movemail-variant-in-use): New variables.
(rmail-remote-password, rmail-remote-password-required): New
customization variables.
(rmail-probe,rmail-autodetect, rmail-movemail-variant-p): New functions
(rmail-parse-url): New function.
(rmail-get-new-mail, rmail-insert-inbox-text): Updated for use
with GNU mailutils movemail.