(IT_clear_end_of_line): Print the extent of the cleared

(IT_clear_end_of_line): Print the extent of the cleared
part of the line to the termscript file.
(IT_clear_to_end): Clear the entire line, not just its beginning.
(menu_help_message, prev_menu_help_message): New variables.
(IT_menu_make_room): Make room for the help_text member.
(IT_menu_display): New argument disp_help; all callers changed.
If disp_help is non-zero, store the help text of the active menu
item in menu_help_message.
(XMenuAddPane): Initialize the help_text member to NULL.
(XMenuAddSelection): New argument help_text.  Store it in the
XMenu structure.
(XMenuActivate): New argument help_callback.  If the value of
menu_help_message has changed since the last time, display the
menu help message text while waiting for the mouse to move.  Clear
the echo area before exiting.
(XMenuDestroy): Free the help_text member.