sip_to_pjsip: improve ability to parse input files

General improvements to SIP to PJSIP conversion utility:

1) track default section of input file to allow parsing
   an include file that doesn't specify a [section]

2) informatively handle case of assignment without [section]

3) correctly handle getting sections from included files
   - [section]'s are inherited by included file

4) provide null string as default transport bind ip

5) gracefully handle missing portions of registration string

6) denote steps of operation during conversion and confirm
   top level files as a convenience

ASTERISK-24474 #close
Review: https://reviewboard.asterisk.org/r/4280/
Reported by: John Kiniston
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Merged revisions 430469 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430470 65c4cc65-6c06-0410-ace0-fbb531ad65f3