The kernel defaults REORDER_HDR to 1 when creating a new VLAN, but
NetworkManager's VLAN flags property defaulted to 0. Thus REORDER_HDR was not
set for NM-created VLANs with default values.

We want to match the kernel default, so we change the default value for the
vlan.flags property. However, we do not want to change the flags for existing
connections if the property is missing in connection files. Thus we have to
update plugins for that. We also make sure that vlan.flags is always written
by 'keyfile' when the value is default. That way new connections have flags
property explicitly written and it will be loaded as expected.

https://bugzilla.redhat.com/show_bug.cgi?id=1250225