MINOR: connection: pass the proxy when creating a connection

Till now it was very difficult for a mux to know what proxy it was
working for. Let's pass the proxy when the mux is instanciated at
init() time. It's not yet used but the H1 mux will definitely need
it, just like the H2 mux when dealing with backend connections.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 780bc57..3faf5ae 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -438,7 +438,7 @@
  * connections from the fact that the context is still NULL. Returns < 0 on
  * error.
  */
-static int h2_init(struct connection *conn)
+static int h2_init(struct connection *conn, struct proxy *prx)
 {
 	if (conn->mux_ctx) {
 		/* we don't support outgoing connections for now */