dsp.c User configuration of DTMF_NORMAL_TWIST and DTMF_REVERSE_TWIST values

Asterisk's DTMF Specifications are based on AT&T specs, which may not be compatible in other countries.

Various countries have different specifications for the maximum power level differences
between the DTMF low group and high group of frequencies.

Power level difference between frequencies for different Administrations/RPOAs
 NTT        = Max. 5 dB
 AT&T       = 4dB(reverse) to 8dB(normal)
 Danish     = Max. 6 dB
 Australian = Max. 10 dB
 Brazilian  = Max. 9 dB
 ETSI       = Max. 6 dB from ETSI ES 201 235-3 V1.3.1 (2006-03)

Now allow 4 variables to be individually configured in dsp.conf, with reasonable min/max of 2dB to 20dB.
Default is AT&T specifications

Add's the following variables to dsp.conf
;dtmf_normal_twist=6.31 
;dtmf_reverse_twist=2.51 
;relax_dtmf_normal_twist=6.31 
;relax_dtmf_reverse_twist=3.98 


(closes issue ASTERISK-20442)
Reported by: tbsky
Tested by: tbsky,alecdavis

alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2141/
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Merged revisions 374384 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374385 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374386 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374387 65c4cc65-6c06-0410-ace0-fbb531ad65f3