BIND 10 #2382: introduce generic version of rdata::createRdata(text)

[BIND 10 Development]

#2382: introduce generic version of rdata::createRdata(text)
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  jinmei
            Priority:  medium        |                       Status:
           Component:  libdns++      |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20121204
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  5             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 trac2382 is ready for in the trac2382 branch.  Note that I pushed the
 same name of branch before and deleted it.  I gave it the same branch
 name assuming almost no one really referred to it.  If the reviewer
 has pulled the previous trac2382 please make sure it doesn't cause
 any confusion.

 This branch is based on a snapshot of #2506 and #2497 and that part
 (up to ca8fc9f) should be ignored for review.

 Other notes:
 - f73f274 is an unrelated bug fix/extension.  See the commit log.
 - 3ae13a8 is another unrelated extension.  I needed this to test
   various cases of the main changes.
 - The rest of the changes are the main part of this task.  I believe
   it's pretty easy to understand.  It's also a quite straightforward
   port of BIND 9's dns_rdata_fromtext() (excluding the part deferred
   to #2523).

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Ticket URL: <http://bind10.isc.org/ticket/2382#comment:5>
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