(free_plist): Cancel the last change.

(free_plist): Cancel the last change.
(write_element): Check if PLIST is nested.
(mplist__from_plist): Label a plist as nested if appropriate.
(mplist__from_alist): Label a plist as nested.
(mplist__conc): Check the value of PL before referring it.  Label
a plist as nested if appropriate.
(mplist_copy): Label a plist as nested if appropriate.
(mplist_put_func): Return Mnil if KEY is a managing key.
(mplist_push): Label a plist as nested if appropriate.