Fix loss of MOH on an ISDN channel when parking a call for the second time.

Using the AMI redirect action to take an ISDN call out of a parking lot
causes the MOH state to get confused.  The redirect action does not take
the call off of hold.  When the call is subsequently parked again, the
call no longer hears MOH.

* Make chan_dahdi/sig_pri restart MOH on repeated AST_CONTROL_HOLD frames
if it is already in a state where it is supposed to be sending MOH.  The
MOH may have been stopped by other means.  (Such as killing the generator.)

This simple fix is done rather than making the AMI redirect action post an
AST_CONTROL_UNHOLD unconditionally when it redirects a channel and thus
potentially breaking something with an unexpected AST_CONTROL_UNHOLD.

(closes issue ABE-2873)
Patches:
      jira_abe_2873_c.3_bier.patch (license #5621) patch uploaded by rmudgett
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Merged revisions 372521 from https://origsvn.digium.com/svn/asterisk/be/branches/C.3-bier
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Merged revisions 372522 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@372523 65c4cc65-6c06-0410-ace0-fbb531ad65f3