Add a "need_carrier" argument to nm_device_is_available(), to allow
distinguishing between "device is not available", "device is fully
available", and "device is available except for not having carrier".

Adjust various parts of NMDevice and NMManager to allow for the
possibility of activating a connection with :carrier-detect = "no" on
a device with no carrier, and to avoid auto-disconnecting devices with
:carrier-detect = "on-activate".

https://bugzilla.gnome.org/show_bug.cgi?id=688284