bridge: Add a vlanfilter bridge option

vlanfilter was originally a per-interface flag to allow more flexible
configurations where some interfaces had VLAN filtering enabled and
some didn't. In practice, this just makes the configuration more
confusing without any real benefit, so remove it, and make vlanfilter
a bridge flag instead.

Add a new bridge option "defuntagged", which sets the automatically
assigned PVID for new members. If set to 0 (the default) then no
PVID is assigned, which matches the current behaviour.

While here, add some more atf_checks to the bridge VLAN tests to
make debugging easier.

Differential Revision: https://reviews.freebsd.org/D51600