Merge open-protocol-stream into open-network-stream.

Merge open-protocol-stream into open-network-stream.

* lisp/subr.el (open-network-stream): Move to net/network-stream.el.

* lisp/gnus/proto-stream.el: Move to net/network-stream.el.

* lisp/net/network-stream.el: Move from gnus/proto-stream.el.
Change prefix to network-stream throughout.
(open-protocol-stream): Merge into open-network-stream, leaving
open-protocol-stream as an alias.  Handle nil BUFFER args.

* lisp/gnus/nnimap.el (nnimap-open-connection-1): Pass explicit :end-of-command
parameter to open-protocol-stream.

* lisp/emacs-lisp/package.el (package--with-work-buffer): Recognize
https URLs.

* lisp/url/url-gw.el (url-open-stream): Use new open-network-stream
functionality to perform encryption.