chan_dahdi: dahdi show channels slices PRI channel dnid on output

dahdi show channels output slices the callerid (which is dnid copied over on
PRI channels). If the channel naming structures look like:

 'DAHDI/i1/1408409XXXX-6'

then the output slices 1408409XXXX down to 1408409XXX. This patch just opens
it up to 15 chars so you can see the whole thing.

(closes issue ASTERISK-22918)
Reported by: outtolunc
Patches:
     svn_chan_dahdi.c.format12_15.diff.txt uploaded by outtolunc (license 5198)
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Merged revisions 404784 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 404785 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 404786 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404787 65c4cc65-6c06-0410-ace0-fbb531ad65f3