device: Remove unmanaged slaves from master when they disappear

We've previously been just watching for state changes into UNMANAGED state. No
state change is emitted upon removal of a device which is already unmanaged.
device: Don't consider device with only LL address configured

LXC domains are given a veth interface that's up and has a LL address
upon their creation.
device,settings: don't assume the device is connected because it's software

It was possibly done to prevent NM from fiddling with the interface. However,
assuming a connection completely prevents the user from being able to use
their conneciton.

Let's just not add default connection to software devices.
Revert "core: make veth devices default-unmanaged for now"

veth devices being default unmanages makes it impossible to configure the
connectivity by NM on container bootup. Now that we don't add a default
connection when a veth is discovered is shouldn't be necessary to prevent
management anymore.

This reverts commit ebeaeaed4d3180c27cc059fe0ff199153fa9ec24.
Merge remote-tracking branches 'fd/lr/ipv6-tokens', 'fd/lr/nm-1-0-fixes', 'fd/lr/unmanaged-slaves', 'fd/lr/device-activation' and 'fd/lr/veth-managed' into integration