Update copyright and commentary.

Update copyright and commentary.
(appt-issue-message): Delete (see appt-activate).
(appt-visible, appt-msg-window): Make obsolete, in favour of
appt-display-format.
(appt-display-mode-line, appt-display-duration)
(appt-display-diary, appt-time-msg-list, appt-mode-string)
(appt-prev-comp-time, appt-display-count, appt-timer)
(appt-convert-time): Doc change.
(appt-disp-window-function, appt-delete-window-function): Use
defcustom rather than defvar.
(appt-display-format): New variable.
(appt-display-message): New function with display code from
appt-check.
(appt-check): Add optional FORCE argument.  Doc change.  Add
appt-make-list to diary-hook if displaying diary.  Remove
checking of view-diary-entries-initially.  Message
display section removed to new function appt-display-message.
(appt-display-window): Doc change.  Remove unused internal var
this-buffer.  Do not beep, since appt-display-message does that.
(appt-make-list): Doc change.  Use caar.
(appt-sort-list): Simplify by using builtin sort function.
(appt-update-list): New function for updating appts when diary is
saved.
(appt-activate): New autoloaded function to toggle package
functionality.