resolv: Implement trust-ad option for /etc/resolv.conf [BZ #20358]

This introduces a concept of trusted name servers, for which the
AD bit is passed through to applications.  For untrusted name
servers (the default), the AD bit in responses are cleared, to
provide a safe default.

This approach is very similar to the one suggested by Pavel Šimerda
in <https://bugzilla.redhat.com/show_bug.cgi?id=1164339#c15>.

The DNS test framework in support/ is enhanced with support for
setting the AD bit in responses.

Tested on x86_64-linux-gnu.

Change-Id: Ibfe0f7c73ea221c35979842c5c3b6ed486495ccc

support/resolv_test.h

@@ -134,6 +134,9 @@ struct resolv_response_flags
   /* If true, the TC (truncation) flag will be set.  */
   bool tc;
 
+  /* If true, the AD (authenticated data) flag will be set.  */
+  bool ad;
+
   /* Initial section count values.  Can be used to artificially
      increase the counts, for malformed packet testing.*/
   unsigned short qdcount;