Only allow 1 6rd or 6to4 interface
Currently it is possible to configure more then 1 interface for 6to4/6rd, however, both use the same stf0 adapter so that won't work. Multi wan routing is also not considered yet (a static route for the proto 41 traffic to the relay is required.).
This means that interfaces.php needs to check if another interface is already using 6rd or 6to4 in $wancfg['ipaddrv6'] and bail if neccesary.