New. An interface for objects that want to act on every message passing

New. An interface for objects that want to act on every message passing

	* libsoup/soup-message-filter.c: New. An interface for objects
	that want to act on every message passing through a session.
	(Initially being used for authentication, but could also be used
	for cache handling, cookie management, etc.)

	* libsoup/soup-connection.c (class_init, etc): Add a message
	filter property.
	(send_request): If the connection has a message filter set, run
	it on the message before sending it.
	(soup_connection_connect_async, etc): When setting up a tunnel, if
	we get back a 407 and the session tries to requeue the message,
	either re-send it, or return SOUP_STATUS_TRY_AGAIN (depending on
	whether or not the proxy closed the connection).
	(soup_connection_connect_sync): Likewise
	(send_request, request_done): Ref/unref the connection

	* libsoup/soup-session.c (soup_session_get_type): Implement the
	SoupMessageFilter interface.
	(soup_session_get_connection): Use the session as the connection's
	message filter
	(soup_session_add_filter, soup_session_remove_filter): Add/remove
	filters from the session
	(setup_message): do auth handling, and call each of the session's
	filters' setup_message methods as well.
	(soup_session_send_message_via): No longer needed.
	(connect_result): Handle SOUP_STATUS_TRY_AGAIN.

	* libsoup/soup-session-async.c (run_queue): Use
	soup_connection_send_request, since soup_session_send_message_via
	is gone now.

	* libsoup/soup-session-sync.c (send_message): Likewise

	* libsoup/soup-message.c (soup_message_is_keepalive): A successful
	response to a CONNECT is always keepalive, even if it's HTTP/1.0
	with no Connection header.

	* libsoup/soup-status.h: add SOUP_STATUS_TRY_AGAIN

	* libsoup/soup-types.h: Add SoupMessageFilter, and macros for
	gobject interface types.

	* tests/get.c (main): Add a -p flag to specify a proxy

	* tests/simple-proxy.c: Fix #includes