MEDIUM: stream-interface: provide a generic si_conn_send_cb callback

The connection send() callback is supposed to be generic for a
stream-interface, and consists in calling the lower layer snd_buf
function. Move this function to the stream interface and remove
the sock-raw and sock-ssl clones.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 844d925..8a4cf3a 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -41,6 +41,7 @@
 int stream_int_shutw(struct stream_interface *si);
 void stream_int_chk_rcv_conn(struct stream_interface *si);
 void stream_int_chk_snd_conn(struct stream_interface *si);
+void si_conn_send_cb(struct connection *conn);
 
 extern struct sock_ops stream_int_embedded;
 extern struct sock_ops stream_int_task;