Merge Emacs' changes.

Merge Emacs' changes.

proto-stream.el (open-protocol-stream): Bring back `network' type.  Make this the default type.
 (proto-stream-open-plain): Rename from proto-stream-open-default.
 (open-protocol-stream, proto-stream-open-starttls, proto-stream-open-tls, proto-stream-open-shell): Replace `default' with `plain'.
nnimap.el (nnimap-stream, nnimap-open-connection-1): Accept `network' value.
nntp.el (nntp-open-connection-function): Document the fact that some values are not functions but are instead handled specially.  Recognize nntp-open-plain-stream value.
 (nntp-open-connection): Recognize that value.