Usage: SET STATUS_CLOCK <text of display for status line clock>
  The value of STATUS_CLOCK is replaced in the STATUS_FORMAT
  variable for any occurrence of %T if CLOCK is ON.  If CLOCK is
  OFF, nothing is displayed for %T in STATUS_FORMAT.  Also, the
  STATUS_CLOCK variable may contain one occurrence of %T which
  will be replaced with the current time of day in the string.
