Adjust `org-at-timestamp-p' behavior

Adjust `org-at-timestamp-p' behavior

* lisp/org.el (org-at-timestamp-p): Change optional argument
  behaviour.  Remove interactive call.
(org-follow-timestamp-link):
(org-get-repeat):
(org-auto-repeat-maybe):
(org-time-stamp):
(org-timestamp-up-day):
(org-timestamp-down-day):
(org-toggle-timestamp-type):
(org-timestamp-change):
(org-goto-calendar):
(org-date-from-calendar):
(org-shiftup):
(org-shiftdown):
(org-shiftright):
(org-shiftleft):
(org-org-menu):
(org-fill-paragraph-with-timestamp-nobreak-p):
(org-shiftcontrolup):
(org-shiftcontroldown):
* lisp/org-agenda.el (org-agenda-date-later):
(org-agenda-date-prompt):
* lisp/org-clock.el (org-clock-timestamps-change):
* lisp/org-mouse.el (org-mouse-delete-timestamp):
(org-mouse-context-menu):
* lisp/org-table.el (org-table-copy-down): Update callers.

* testing/lisp/test-org.el (test-org/at-timestamp-p): Add tests.