Parse new address message

Whenever a new address is assigned to an interface we are notified. The
message contains (among others) the new IP address, the interface label
and interface name (TODO!).

We currently only handle IPv4, but handling IPv6 is only a minor
extension.
Check label size

The interface label appears to simply be the interface name (although
we'll try not to rely on that), so assert that it'll never be longer
than IFNAMSIZ.
Trivial: Kill unneeded empty line
Retrieve the interface name.

We can rely on the ifindex to point to the correct interface, so use
that as the source for the 'interfaceName'.