If a device had its carrier ignored, and did not have a carrier on startup,
then NetworkManager would not re-check autoconnect connections when the
device's carrier appeared.  Because ignore-carrier devices are always
in DISCONNECTED state when they are managed, the nm-device.c::carrier_changed()
code essentially did nothing when the carrier appeared.  It needs to
also trigger an auto-activation recheck signal when the carrier appears
to ensure that now-valid connections (like those that require DHCP or
IPv6) can be auto-activated.