BIND 10 #2369: InputSource helper class for MasterLexer

[BIND 10 Development]

#2369: InputSource helper class for MasterLexer
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                   Reporter:         |                 Owner:  muks
  jinmei                             |                Status:  reviewing
                       Type:  task   |             Milestone:
                   Priority:         |  Sprint-20121106
  medium                             |            Resolution:
                  Component:         |             Sensitive:  0
  libdns++                           |           Sub-Project:  DNS
                   Keywords:         |  Estimated Difficulty:  4
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
  loadzone-ng                        |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Comment (by jinmei):

 In my attempt of integrating this branch into #2371, I noticed a few
 more things:

 - I suggest including what's wrong when throwing OpenError:
 {{{#!cpp
         isc_throw(OpenError,
                   "Error opening the input source file: " << filename);
 }}}
   "no such file", "permission denied", etc.
 - if not super hard, I'd test a different failure case for `OpenError`
   like it's not readable (due to permission or possible some other
   reason)
 - In `InputSource::getChar` variables `c` should become const.  Also
   check other possible cases where you can make them const.

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Ticket URL: <http://bind10.isc.org/ticket/2369#comment:16>
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