Add support for configuring vlan(4) interfaces as child devices similar to
wlan(4) interfaces. vlan(4) interfaces are listed via a new 'vlans_<IF>'
variable. If a vlan interface is a number, then that number is treated as
the vlan tag for the interface and the interface will be named '<IF>.<tag>'.
Otherwise, the vlan tag must be provided via a vlan parameter in a
'create_args_<vlan>' variable.
While I'm here, fix a few nits in rc.conf(5) and mention create_args_<IF> in
the description of cloned_interfaces.
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