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.c

@@ -182,6 +182,8 @@ resolv_response_init (struct resolv_response_builder *b,
   if (flags.tc)
     b->buffer[2] |= 0x02;
   b->buffer[3] = 0x80 | flags.rcode; /* Always set RA.  */
+  if (flags.ad)
+    b->buffer[3] |= 0x20;
 
   /* Fill in the initial section count values.  */
   b->buffer[4] = flags.qdcount >> 8;