Move some duplicated carrier-handling code into NMDevice (which can
introspect itself to see if it's a subclass that has carrier).
The "mostly ignore carrier" special handling for bridges and bonds is
now also handled as part of the NMDevice-level carrier handling.
https://bugzilla.gnome.org/show_bug.cgi?id=688284
The code flow is actually somewhat simpler this way since the
subclasses don't have to ask NMDeviceWired for the address
every time. Plus then NMDeviceWired doesn't have to know
anything about its subclasses in the constructor.
Speed is gotten via ethtool on 'carrier on' event. If there is a more suitable
netlink event we should use it instead. Nonetheless, it appears to be working
fine with carrier change, because interface speed change does emit carrier offi
and on events.