Certain types of interface were excluded from bcp38's concept of
upstream wan interface e.g. pppoa-wan
Shamelessly copies the interface list generation from luci-app-sqm which
solved exactly the same problem and gives additional clues by displaying
the firewall zone (typically wan/lan etc)
Run-tested-on: DGN3500
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
