freetdm: * Avoid acquiring the span mutex if the pendingchans queue is available...

freetdm: * Avoid acquiring the span mutex if the pendingchans queue is available for use *
         * FreeTDM modules using the old FTDM_SPAN_STATE_CHANGE flag should be updated   *
         * until then, they are still vulnerable to deadlock situations                  *
         * Modules pending update: (ftmod_analog, ftmod_libpri, ftmod_isdn)              *
         * Fixes Sangoma redmine ticket #1791 0 FTDM span stop deadlock                  *