Make sieve-open-server respect the PORT argument
Show the correct port number in the sieve-buffer's header
Set sieve-manage-default-port to 4190

The IANA allocated port for ManageSieve is 4190, the old port 2000 is
reserved for Cisco SCCP.
Make sieve-manage-open work with STARTTLS

Shorten stream managing functions in sieve-manage by using
open-protocol-stream to do most of the work.  Has the nice benefit of
enabling STARTTLS.
Remove unneeded functions and option

Delete the following functions, as they are neither in the API, nor
called by any other function:

* sieve-manage-network-p
* sieve-manage-network-open
* sieve-manage-starttls-p
* sieve-manage-starttls-open
* sieve-manage-forward

Delete options sieve-manage-streams and sieve-manage-stream-alist, as
those cannot be applied in a meaningful way; they didn't happen to have
much effect before, anyway.
Enforce a clean buffer when calling sievel-sasl-auth
Clean the sieve process buffer setup code

Split into separate function to make it clearer what's going on in the
code.
Replace sieve-manage-disable-multibyte with mm-disable-multibyte

Having a custom macro just confuses things, make use of the respective
mm-util utility function instead.
Do minor sieve-manage cleanup