Initialize the switch vlan table at attachment.

Update some comments on code, specifying the correct vlans used on switch
setup.

Advertise the proper switch operation mode (the rtl8366rb only support
dot1q vlans).

This fixes the breakage that i introduced on r249752 and make the rtl8366rb
switch works again with etherswitchcfg(8).

Tested on TP-Link 1043ND.

Tested by:	me, Harm Weites (harm at weites.com)
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)