Properly advertise that if_arge can handle long frames (if_arge is set to

Properly advertise that if_arge can handle long frames (if_arge is set to
handle packets up to 1536 bytes)

This fixes the need to frag that could happen when using vlans on top of
if_arge (which is a common case for the use the switch ports as individual
NICs).

Previously to this commit any vlan setup with if_arge as parent would have
the MTU of the parent interface reduced by the size of dot1q header
(4 bytes).

Tested on TP-Link 1043ND (where the WAN port is just a switch port setup to
tag packets in a different VLAN than the LAN ports).

Reported and tested by:	Harm Weites (harm at weites.com)