Port range support in the firewaller.

The firewaller watches the openedPorts collection, the ports field on units is no longer updated.

This work fixes https://bugs.launchpad.net/juju-core/+bug/1337817
*is* it used by various facades? maybe one day it will be used by the machine, but I'm not 100% sure it's justified yet.

Not really bothered by that, but found myself looking at it again when I saw its name in a different context and got really confused: this is certainly not any sort of Watcher :).