Fix a longstanding issue with MFC-R2 configuration that prevented users
from mixing different variants or general MFC-R2 settings within the same E1 line.

Most users do not have a problem with this since MFC-R2 lines are usually fractional E1s, or
the whole E1 has the same country variant and R2 settings.

In Venezuela however is common to have inbound MFC-R2 and outbound DTMF-R2 within the same E1.

This fix now properly parses the chan_dahdi.conf file to generate a new openr2 context every
time a new channel => section is found and the configuration was changed.

(closes issue ASTERISK-21117)
Reported by: Rafael Angulo
Related Elastix issue: http://bugs.elastix.org/view.php?id=1612
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Merged revisions 394106 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 394173 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@394263 f38db490-d61c-443f-a65b-d21fe96a405b