MAJOR: connection: make the PROXY decoder a handshake handler

The PROXY protocol is now decoded in the connection before other
handshakes. This means that it may be extracted from a TCP stream
before SSL is decoded from this stream.
diff --git a/include/proto/connection.h b/include/proto/connection.h
index 007f9e2..a575e69 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -30,6 +30,9 @@
  */
 int conn_fd_handler(int fd);
 
+/* receive a PROXY protocol header over a connection */
+int conn_recv_proxy(struct connection *conn, int flag);
+
 /* calls the init() function of the data layer if any. Returns <0 in case of
  * error.
  */