Allow backtrace to work before the load of the config file

Allow backtrace to work before the load of the config file

- working_directory is NULL until the config file has been loaded
- there was a bug in signal_handler(), working_directory (wd) was set to
  "buf" and we were doing a
  snprintf(buf, sizeof(buf), "%s/bacula.%s.traceback", wd, pid_buf);
  where wd and buf point to the same location, that was generating
  funny path like /bacula.7647.traceback/bacula.7647.traceback
- now we get something like /tmp/bacula.31150.traceback
- no need of any check in "btraceback"