Change default type of open-protocol-stream.

Change default type of open-protocol-stream.

* 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.

* 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'.