samba_dnsupdate: honor 'dns zone scavenging' option, only update if needed

Since scavenging is implemented the samba_dnsupdate command always updates all
dns records required by the dc. This is not needed if dns zone scavenging
is not enabled.

This avoids the repeating TSIG error messages:

 # samba_dnsupdate --option='dns zone scavenging = yes' 2>&1 | uniq -c
     29 ; TSIG error with server: tsig verify failure
      1 Failed update of 29 entries
 # echo ${PIPESTATUS[0]}
 29

 # samba_dnsupdate --option='dns zone scavenging = no' 2>&1 | uniq -c
 # echo ${PIPESTATUS[0]}
 0

Note that this results in about 60 lines in the log file,
which triggered every 10 minutes ("dnsupdate:name interval=600" is the default).

This restores the behavior before 8ef42d4d,
if "dns zone scavenging" is not switched on (which is still the default).

Avoiding the message from happening at all is subject for more debugging,
most likely they are caused by bugs in 'nsupdate -g' (from the bind package).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13605

Pair-programmed-with: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Björn Baumbach <bb@sernet.de>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
22 jobs for metze-autobuild3 in 373 minutes and 46 seconds (queued for 2 seconds)