dsp.c: Fix erroneous fax tone detection.

The Goertzel calculations get less accurate the lower the signal level
being worked with becomes because there is less resolution remaining.
If it is too low we can erroneously detect a tone where none really
exists.  The searched for fax frequencies not only need to be so much
stronger than the background noise they must also be a minimum strength.

* Add needed minimum threshold test to tone_detect().

* Set TONE_THRESHOLD to allow low volume frequency spread detection.

ASTERISK-26237 #close
Reported by: Richard Mudgett

Change-Id: I84dbba7f7628fa13720add6a88eae3b129e066fc