Explicitely cast Conf_MaxPenaltyTime (time_t) to "long"

This fixes the following compiler warning, for example on OpenBSD:

  conf.c: In function 'Conf_Test':
  conf.c:391: warning: format '%ld' expects type 'long int', but argument
                       2 has type 'time_t'

Thanks to Götz Hoffart for reporting this!