Add wrote_informational and got_informational signals.

Add wrote_informational and got_informational signals.

	* libsoup/soup-message.c: Add wrote_informational and
	got_informational signals.

	* libsoup/soup-message-client-io.c (get_request_headers): Set the
	EXPECT_CONTINUE flag on the message if that header is set.

	* libsoup/soup-message-server-io.c (parse_request_headers):
	Likewise

	* libsoup/soup-message-io.c (io_write): Set read_state to HEADERS
	when blocking on an expect-continue. Emit wrote_informational
	instead of wrote_headers in the 1xx case.
	(io_read): Set read_state to BLOCKING, not NOT_STARTED after
	reading a 100 Continue response. Emit got_informational instead of
	got_headers in the 1xx case.

	* libsoup/soup-session.c (soup_session_send_message): Reorder
	things to deal with the fact that the message could finish right
	away if there is a connection available and the server is very
	close.

	* libsoup/soup-status.h: Rename SOUP_STATUS_CLASS_TRANSPORT to
	SOUP_STATUS_CLASS_TRANSPORT_ERROR.