MINOR: mux/server: Add 'proto' keyword to force the multiplexer's 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 server's definition.
diff --git a/include/types/server.h b/include/types/server.h
index c30003b..7c6d225 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -205,6 +205,7 @@
 	char *rdr_pfx;				/* the redirection prefix */
 
 	struct proxy *proxy;			/* the proxy this server belongs to */
+	struct mux_proto_list *mux_proto;       /* the mux to use for all outgoing connections (specified by the "proto" keyword) */
 	int served;				/* # of active sessions currently being served (ie not pending) */
 	int cur_sess;				/* number of currently active sessions (including syn_sent) */
 	unsigned maxconn, minconn;		/* max # of active sessions (0 = unlimited), min# for dynamic limit. */