The radius server, when denying authorization, may reply as follows:

   Received response ID 79, code 3, length = 44
       Tunnel-Type:0 = 13
       Tunnel-Medium-Type:0 = IEEE-802
       Framed-Protocol = PPP
       Service-Type = Framed-User

i.e., it misses the vlan number as "Tunnel-Private-Group-Id", while
everything else is in place. In this case we should not obey
dotconfig, but rather keep the noauth_vlan.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>