Get rid of NM_UNMANAGED_DEFAULT and refine the interaction between
unmanaged flags, device state and managed property.

Previously, the NM_UNMANAGED_DEFAULT was special in that a device was
still considered managed if it had solely the NM_UNMANAGED_DEFAULT flag
set and its state was managed. Thus, whether the device (state) was managed,
depended on the device state too.

Now, a device is considered managed (or unmanaged) based on the unmanaged
flags and realization state alone. At the same time, the device state
directly corresponds to the managed property of the device. Of course,
while changing the unmanaged flags, that invariant is shortly violated
until the state transistion is complete.

Introduce more unmanaged flags whereas some of them are non-authorative.
For example, the EXTERNAL_DOWN flag has only effect as long as the user
didn't explicitly manage the device (NM_UNMANAGED_USER_EXPLICIT). In other
words, certain flags can render other flags ineffective. Whether the device
is considered managed depends on the flags but also at the explicitly unset flags.
In a way, this is similar to previous where NM_UNMANAGED_DEFAULT was ignored
(if no other flags were present).

Also, previously a device that was NM_UNMANAGED_DEFAULT and in disconnected
state would transition back to unmanaged. No longer do that. Once a device is
managed, it stays managed as long as the flags indicate it should be managed.
However, the user can also modify the unmanaged flags via the D-Bus API.

Also get rid or nm_device_finish_init(). That was previously called
by NMManager after add_device(). As we now realize devices (possibly
multiple times) this should be handled during realization.

https://bugzilla.gnome.org/show_bug.cgi?id=746566