net/iodine: make port configurable

Fall back to the default DNS port (i.e. 53). If you configure something
different than 53 this results in a warning from iodined:

	ALERT! Other dns servers expect you to run on port 53.
	You must manually forward port 53 to port $port for things to work.

This is obviously true.

Signed-off-by: Uwe Kleine-König <uwe+openwrt@kleine-koenig.org>
net/iodine: make address config optional

In the absense of an address entry bind to all interfaces which is also
iodined's default when -l isn't given.

Signed-off-by: Uwe Kleine-König <uwe+openwrt@kleine-koenig.org>
Merge pull request #125 from ukleinek/iodine

net/iodine: config updates