BIND 10 #2021: make config handler of b10-ddns more friendly

[BIND 10 Development]

#2021: make config handler of b10-ddns more friendly
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  very low      |                    Milestone:
           Component:  DDNS          |  Sprint-20120612
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:  DDNS
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a deferred task from #2013.

 The config handler for b10-ddns (the only configurable thing is ACL
 right now) is pretty primitive and not very user friendly:

 - It doesn't indicate which part of the given configuration is wrong
   (e.g., try adding multiple ACLs for several zones, including one
   typo in a zone name so it won't be a valid domain name; or even
   leave it as an empty string).  It should be more helpful by being as
   specific as possible.  Such a problem actually exists in other
   applications, so we might even consider some generalized framework
   and apply it here (but it may be a subject of a bigger task of
   config cleanups)
 - Duplicate zone (ACL) configuration is currently just ignored. It's
   probably better to reject it or at least make a local log a
   message (see also the test_config_handler test, searching for
   "duplicate")

 I'm pushing it to the current sprint with a lower priority in case we
 run out of open tasks.  But I don't think this is a critical missing
 feature, so it can wait.

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Ticket URL: <http://bind10.isc.org/ticket/2021>
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