netdev-dummy: add appctl netdev-dummy/conn-state command

Using without any parameter, this command list the connection
state of all netdev-dummy devices that are configured to make
active connections.

Optionally, the name of the netdev-dummy device can be supplied
as a parameter.

The states will be displayed as:

"connected": The socket has been connected to the listener.
"disconnected": The socket is not connected.
"unknown":  It is not an active dummy device.

CC: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>