History List Management
-----------------------

   These functions manage individual entries on the history list, or set
parameters managing the list itself.

 - Function: void add_history (char *string)
     Place STRING at the end of the history list.  The associated data
     field (if any) is set to `NULL'.

 - Function: HIST_ENTRY * remove_history (int which)
     Remove history entry at offset WHICH from the history.  The
     removed element is returned so you can free the line, data, and
     containing structure.

 - Function: HIST_ENTRY * replace_history_entry (int which, char *line,
          char *data)
     Make the history entry at offset WHICH have LINE and DATA.  This
     returns the old entry so you can dispose of the data.  In the case
     of an invalid WHICH, a `NULL' pointer is returned.

 - Function: void clear_history ()
     Clear the history list by deleting all the entries.

 - Function: void stifle_history (int max)
     Stifle the history list, remembering only the last MAX entries.

 - Function: int unstifle_history ()
     Stop stifling the history.  This returns the previous amount the
     history was stifled.  The value is positive if the history was
     stifled, negative if it wasn't.

 - Function: int history_is_stifled ()
     Returns non-zero if the history is stifled, zero if it is not.