Changes to open-protocol-stream, preparing for merging it with open-network-stream.

Changes to open-protocol-stream, preparing for merging it with open-network-stream.

* lisp/gnus/proto-stream.el: Changes preparatory to merging open-protocol-stream
with open-network-stream.
(proto-stream-always-use-starttls): Option removed.
(open-protocol-stream): Return a process object by default.  Provide a
new parameter :return-list specifying a list-type return value, which
now has the form (PROP . PLIST) instead of a fixed-length list.  Change
:type `network' to `try-starttls', and `network-only' to `default'.
Make `default' the default, for compatibility with open-network-stream.
Handle the no-parameter case exactly as open-network-stream, with no
additional stream processing.  Search plists using plist-get.
Explicitly add :end-of-commend parameter if it is missing.
(proto-stream-open-default): Renamed from
proto-stream-open-network-only.  Return 'default as the type.
(proto-stream-open-starttls): Rename from proto-stream-open-network.
Use plist-get.  Don't return `tls' as the type if STARTTLS negotiation
failed.  Always return a list with a (possibly dead) process as the
first element, for compatibility with open-network-stream.
(proto-stream-open-tls): Use plist-get.  Always return a list.
(proto-stream-open-shell): Return `default' as connection type.
(proto-stream-capability-open): Use plist-get.
(proto-stream-eoc): Function deleted.

* lisp/gnus/nnimap.el (nnimap-stream, nnimap-open-connection)
(nnimap-open-connection-1): Handle renaming of :type parameter for
open-protocol-stream.
(nnimap-open-connection-1): Pass a :return-list parameter
open-protocol-stream to obtain a list return value.  Parse this list
using plist-get.

* lisp/gnus/nntp.el (nntp-open-connection): Handle renaming of :type parameter
for open-protocol-stream.  Accept open-protocol-stream return value
that is a subprocess object instead of a list.  Handle the case of a
dead returned process.