lisp/gnus/sievel-manage.el: fully support STARTTLS, fix bit rot

lisp/gnus/sievel-manage.el: fully support STARTTLS, fix bit rot

* Make sieve-manage-open work with STARTTLS: shorten stream managing
  functions by using open-protocol-stream to do most of the work.  Has
  the nice benefit of enabling STARTTLS.

* Remove unneeded functions and options: the following functions and
  options are neither in the API, nor called by any other function, so
  they are deleted:
  - sieve-manage-network-p
  - sieve-manage-network-open
  - sieve-manage-starttls-p
  - sieve-manage-starttls-open
  - sieve-manage-forward
  - sieve-manage-streams
  - sieve-manage-stream-alist

  The options could not be applied in a meaningful way anymore; they
  didn't happen to have much effect before.

* Cosmetic changes and code clean-up

* Enable Multibyte for SieveManage buffers: The parser won't properly
  handle umlauts and line endings unless multibyte is turned on in the
  process buffer.

* Wait for capabilities after STARTTLS: following RFC5804, the server
  sends new capabilities after successfully establishing a TLS
  connection with the client.  The client should update the cached list
  of capabilities, but we just ignore the answer for now.