Add :coding support to open-network-stream and open-gnutls-stream

Add :coding support to open-network-stream and open-gnutls-stream

* doc/lispref/processes.texi (Network): Describe :coding keyword support.

* doc/misc/emacs-gnutls.texi (Help For Developers): Describe :coding
keyword support.

* etc/NEWS: Announce change to open-network-stream and
open-gnutls-stream.

* lisp/net/gnutls.el (open-gnutls-stream): Add support for :coding, pass it
down to open-network-stream.

* lisp/net/network-stream.el (open-network-stream)
(network-stream-open-plain, network-stream-open-starttls): Add
support for :coding, pass it down to make-network-process.
(network-stream-open-shell): Add support-for :coding, use
set-process-coding-system to set it after process creation.