contrib/freifunk-gwcheck: check which interface is wan when we get the upstream dns servers, add priorities to logger calls, check for dyngw_plain plugin before checking if script already runs, add license/copyright

git-svn-id: http://svn.luci.subsignal.org/luci/trunk@9625 ab181a69-ba2e-0410-a84d-ff88ab4c47bc