Add 'nat_devices': a list of devices that will be natted (beside dsl devices)

conf/interfaces_ipv4.conf.sample

@@ -36,6 +36,14 @@ local_if_7=""
 
 local_ifs="$local_if_1 $local_if_2 $local_if_3 $local_if_4 $local_if_5 $local_if_6 $local_if_7"
 
+# - Devices given in list "nat_devices" will be natted
+# -
+# - Notice: Devices "ext_if_dsl_n" will be natted and must not been given here.
+# -
+# - Blank separated list
+# -
+nat_devices=""
+
 # - Are local alias interfaces like eth0:0 defined"
 # -
 local_alias_interfaces=true