shill: Add singleton to output resolver files

Output a file that can be linked to "resolv.conf" from the contents
of an ipconfig file.

BUG=chromium-os:17278
TEST=New unittest

Change-Id: Ic537dda6f1cb337d631bc75a3aaa835312ce8c01
Reviewed-on: http://gerrit.chromium.org/gerrit/5399
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/ipconfig.h b/ipconfig.h
index e892c3d..ca922a3 100644
--- a/ipconfig.h
+++ b/ipconfig.h
@@ -96,6 +96,8 @@
   FRIEND_TEST(DeviceTest, DestroyIPConfig);
   FRIEND_TEST(IPConfigTest, UpdateCallback);
   FRIEND_TEST(IPConfigTest, UpdateProperties);
+  FRIEND_TEST(ResolverTest, Empty);
+  FRIEND_TEST(ResolverTest, NonEmpty);
   FRIEND_TEST(RoutingTableTest, RouteAddDelete);
 
   static const char kStorageType[];