Fix the ability to specify an OSP token for an outbound IAX2 call.

When this patch was originally submitted, the code allowed for the token to be
set via a channel variable.  I decided that a cleaner approach would be to
integrate it into the CHANNEL() function.  Unfortunately, that is not a suitable
approach.  It's not possible to get the value set on the channel soon enough
using that method.  So, go back to the simple channel variable method.

(closes issue #16711)
Reported by: homesick
Patches:
      iax-svn.diff uploaded by homesick (license 91)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@243482 65c4cc65-6c06-0410-ace0-fbb531ad65f3