unify permissions check for VLAN changes

The VLAN permissions logic from authorize8021QChangeRequests
(functions.php) was duplicated in getAccessPortControlCode
(interface.php). New function nativeVlanChangePermitted now used to
check access ports VLAN permissions. It calls the
authorize8021QChangeRequests in turn.

Also, the overriding hook added into the authorize8021QChangeRequests.