Now it is user-controlled through ifconfig(8).
The former ``automagic'' way of operation created more
trouble than good. First, VLAN_MTU consumers other than
vlan(4) had appeared, e.g., ng_vlan(4). Second, there was
no way to disable VLAN_MTU manually if it were causing
trouble, e.g., data corruption.
Dropping the ``automagic'' should be completely invisible
to the user since
a) all the drivers supporting VLAN_MTU
have it enabled by default, and in the first place
b) there is only one driver that can really toggle VLAN_MTU
in the hardware under its control (it's fxp(4), to which
I added VLAN_MTU controls to illustrate the principle.)
d6fcfb7ae1