[Dovecot] Patch: support URLAUTH, BURL, CATENATE

[Mike Abbott]

> That says the submit server's IMAP client must support STARTTLS+SASL
> PLAIN authentication.

Yes.

> Which is exactly the opposite of what X-PLAIN-SUBMIT does.

X-PLAIN-SUBMIT is intended to allow a submission server to connect to the IMAP server, send STARTTLS, and then authenticate using a plain-text password -- even when PLAIN is disabled for normal users.  Nothing more.  We just didn't want URLAUTH to break when the administrator shuts down PLAIN auth.