Lay the groundwork for enabling EDNS0 in queries.

This change does not enable EDNS0, so it should not
result in any behavior change.  However, enabling EDNS0
should now be possible with only a small additional change
to "flip the switch".

This change has also landed in NetBSD upstream
(http://gnats.netbsd.org/52578) so this change reduces
divergence from upstream.

Most of the code in this change is for caching of queries that contain
an additional section.

Bug: 15132200
Test: Added integration tests for fallback to the netd suite.
Change-Id: Ic64bed0754e1d529dc0c0ab6a5e2f1ea201ff0d5