Add MFC/R2 support for chan_dahdi.

This commit introduces official support for R2 signaling in chan_dahdi.  The
modifications to chan_dahdi, and the supporting library, LibOpenR2, were both
written by Moises Silva.

Many users are using this code, or a variant of it, in Asterisk 1.2, 1.4 and 1.6
in Brazil, México and Argentina. An unknown number of users (but at least 1) 
are using it in each of the following countries: Colombia, Nepal, Thailand, 
Venezuela, Perú, and probably others.

To use this code, LibOpenR2 must be installed from http://www.libopenr2.org/.
Information about configuration can be found in configs/chan_dahdi.conf.sample.

The code committed is the most up to date version, which was being maintained
in svn/asterisk/team/moy/mfcr2/.

I would also like to include a Thank You to the many others that tested this
code beyond those listed in this commit message.  These are the names that I
could find in the mantis issue.

(closes issue #12509)
Reported by: moy
Patches:
      chan_zap-mfr2.patch uploaded by moy (license 222)
Tested by: moy, korihor, viniciusfontes, Skarmeth, loloski, asbestoshead, titogarrido, heliocoelhojr, konsultex, ncorrare, ecarruda, rtorresduque, PTorres, ychen

Review: http://reviewboard.digium.com/r/40/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@182355 65c4cc65-6c06-0410-ace0-fbb531ad65f3