Add protection for SS7 channel allocation and better glare handling.

* Added a CLI "ss7 show channels" command that might prove useful for
future debugging.

* Made the incoming SS7 channel event check and gripe message uniform.

* Made sure that the DNID string for an incoming call is always
initialized.

(issue ASTERISK-17966)
Reported by: Kenneth Van Velthoven
Patches:
      jira_asterisk_17966_v1.8_glare.patch (license #5621) patch uploaded by rmudgett
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Merged revisions 340365 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 340366 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@340367 65c4cc65-6c06-0410-ace0-fbb531ad65f3