ifconfig: Support VLAN ID in static/deladdr

Add an optional "vlan <n>" argument to the bridge static and deladdr
commands to allow addresses to be added to / removed from a particular
vlan. No changes to if_bridge are required as the kernel API already
supports this, it just wasn't exposed in ifconfig.

Add tests for the new functionality, and improve the test for the
existing "static" command.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D51243