Add dynamic fax buffer configuration option to chan_dahdi.conf

When the 'faxdetect' configuration option is used, one may also want to use
the 'faxbuffers' configuration option in chan_dahdi.conf.  This option will
dynamically use the configured 'faxbuffers' buffer policy on a channel for
the life of the call following the detection of fax tones.  The faxbuffers
buffer policy will be reverted during call teardown.

An example use of 'faxbuffers' is below.  This example would switch to using
6 buffers with a full buffer policy.

faxbuffers=>6,full


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@175411 65c4cc65-6c06-0410-ace0-fbb531ad65f3