Sample output:

	# ./bridge/bridge mdb show dev br0
	bridge br0 port eth1 group 224.8.8.9
	bridge br0 port eth0 group 224.8.8.8

	# ./bridge/bridge -d mdb show dev br0
	bridge br0 port eth1 group 224.8.8.9
	bridge br0 port eth0 group 224.8.8.8
	router ports on br0: eth0
Signed-off-by: Cong Wang <amwang@redhat.com>