Instead of a recompile, allow values to be adjusted in dsp.conf

For binary distributions allows easy adjustment for wobbly GSM calls, and other reasons.

Defaults to DTMF_HITS_TO_BEGIN=2 and DTMF_MISSES_TO_END=3

(closes issue ASTERISK-17493)
Reported by: alecdavis
Tested by: alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2144/
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Merged revisions 374479 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374481 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374485 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374493 65c4cc65-6c06-0410-ace0-fbb531ad65f3