Add the new configuration option 'assume-ipv6ll-only' which specifies
the devices for which NM will try to assume an existing IPv6LL-only
configuration.

The new default behavior is to ignore such configurations since IPv6LL
addresses are automatically assigned by the kernel when the device is
brought up and thus the presence of an IPv6LL address doesn't mean
that the device was configured by the administrator.

The previous behavior was to always assume IPv6LL-only configurations
but this often had the unwanted effect of preventing other on-disk
configurations to be activated. To preserve the old behavior the
option must be set to '*'.

https://bugzilla.redhat.com/show_bug.cgi?id=1138426