The ifuntagged option was added as part of the VLAN filtering feature,
but it's useful on its own to be able to place interface traffic in a
VLAN without having to configure every interface for VLAN filtering.

Always do the pvid processing in bridge even if IFBRF_VLANFILTER isn't
enabled, and don't prohibit configuring it.

Add a test for the specific case of setting untagged without vlanfilter.

This has no effect on bridges which don't have at least one interface
configured with ifuntagged.