More changes from chan_dahdi that did not make it into sig_pri.

*  Q.SIG channel mapping option.
*  discardremoteholdretrieval option.
*  libPRI debug defines.
*  pri_set_overlapdial() now set correctly.
*  pthread creation of pri_ss_thread now matches.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@210696 65c4cc65-6c06-0410-ace0-fbb531ad65f3