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: voidclear_history ()
Clear the history list by deleting all the entries.
Function: voidstifle_history (int max)
Stifle the history list, remembering only the last max entries.
Function: intunstifle_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: inthistory_is_stifled ()
Returns non-zero if the history is stifled, zero if it is not.