Reported by Petr Sumbera <petr.sumbera@oracle.com>
Two threads entering xmlInitializeDict concurently could lead
to a lockup due to multiple initializations of the lock used.
To avoid this problem move this to a new private function
called from xmlOnceInit() and deprecate the old initalizer.
Since threaded programs must call xmlInitParser() and this
will lead to dereference of private data and the call to
xmlOnceInit() guaranteed to be unique this should be safe now.