Comment 325 for bug 191889

Changes to the previous patch:
- the function nm_device_other_update_ppp is changed to nm_device_update_unmanaged and defined in the header file
- now it monitors all unmanaged devices instead of just ppp devices
- now it checks whether the unmanaged device have an IP address before signaling as online
- added a new flag unmanaged_active (mimicking modem_active) that store whether we are online or not
- (untested) handle cases where there are more than one unmanaged devices and one of them is online. (This becomes an issue after I switched to netlink)