BIND 10 #1260: add disable IXFR configuration option add xfrin

[BIND 10 Development]

#1260: add disable IXFR configuration option add xfrin
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  blocker       |                    Milestone:
           Component:  xfrin         |  Sprint-20111011
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a subtask of #1212 and can be done independently from other
 subtasks.

 The idea is to try IXFR by default (no fall back to AXFR for now) and
 an option to disable it (and use AXFR from the beginning and only try
 AXFR in that case).  Preferably it's configurable per zone and/or
 server address, but can also be global.

 In this small subtask, we only update the spec file to support the new
 option and update the option handler accordingly.

 In the worst case if we don't have time maybe we can defer it after
 the next release.

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Ticket URL: <http://bind10.isc.org/ticket/1260>
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BIND 10 Development