bug #10609: CVE-2014-0239 Don't reply to replies

bug #10609: CVE-2014-0239 Don't reply to replies

Due to insufficient input checking, the DNS server will reply to a packet that
has the "reply" bit set. Over UDP, this allows to send a packet with a spoofed
sender address and have two servers DOS each other with circular replies.

This patch fixes bug #10609 and adds a test to make sure we don't regress.
CVE-2014-2039 has been assigned to this issue.
Bug: https://bugzilla.samba.org/show_bug.cgi?id=10609

Signed-off-by: Kai Blin <kai@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Kai Blin <kai@samba.org>
Autobuild-Date(master): Tue May 20 04:15:44 CEST 2014 on sn-devel-104

(cherry picked from commit 392ec4d241eb19c812cd49ff73bd32b2b09d8533)

Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Mon May 26 14:54:32 CEST 2014 on sn-devel-104