chan_dahdi: Don't ignore setvar when using configuration section scheme.

chan_dahdi: Don't ignore setvar when using configuration section scheme.

When the configuration section scheme of chan_dahdi.conf is used (keyword
dahdichan instead of channel) all setvar= options are completely ignored.
No variable defined this way appears in the created DAHDI channels.

* Move the clearing of setvar values to after the deferred processing of
dahdichan.

AST-1378 #close
Reported by: Guenther Kelleter
Patch by: Guenther Kelleter
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Merged revisions 429825 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 429829 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/13.1@429860 65c4cc65-6c06-0410-ace0-fbb531ad65f3