MINOR: mux/frontend: Add 'proto' keyword to force the mux protocol

For now, it is parsed but not used. Tests are done on it to check if the side
and the mode are compatible with the proxy's definition.
diff --git a/include/types/listener.h b/include/types/listener.h
index c55569c..09b1104 100644
--- a/include/types/listener.h
+++ b/include/types/listener.h
@@ -156,6 +156,7 @@
 	EVP_PKEY *ca_sign_pkey;    /* CA private key referenced by ca_key */
 #endif
 	struct proxy *frontend;    /* the frontend all these listeners belong to, or NULL */
+	struct mux_proto_list *mux_proto; /* the mux to use for all incoming connections (specified by the "proto" keyword) */
 	struct xprt_ops *xprt;     /* transport-layer operations for all listeners */
 	int is_ssl;                /* SSL is required for these listeners */
 	int generate_certs;        /* 1 if generate-certificates option is set, else 0 */