dns_dlzcreate() failed to properly initialize
                        dlzdb.link.  When cloning a rdataset do not copy
                        the link contents.  [RT #32651]

Squashed commit of the following:

commit c36c49cbdaeec8b2506dffadbffa543283702fa2
Author: Mark Andrews <marka@isc.org>
Date:   Mon Feb 18 23:24:57 2013 +1100

    don't copy the link when cloning a rdataset

commit 9fef5827edcc925075832dcce900eeca9057456d
Author: Mark Andrews <marka@isc.org>
Date:   Mon Feb 18 23:23:25 2013 +1100

    initialise the dlzdb link; don't return a stale pointer on error

commit a13c584732eae2dde48920a73886b54f1fe6b030
Author: Mark Andrews <marka@isc.org>
Date:   Mon Feb 18 23:21:59 2013 +1100

    turn on ISC_LIST_CHECKINIT