[PATCH] Use `org-clock-string' whenever possible

* lisp/org-clock.el (org-find-open-clocks):
* lisp/org.el (org-clone-subtree-with-time-shift,
 org-insert-property-drawer, org-at-clock-log-p): Use
 `org-clock-string' whenever possible instead of hardcoded "CLOCK".
 Add the `org-clock-line-re' regexp matching a line with clock info.