(read_mtext_element): Ignore the sequence of '\\' and

(read_mtext_element): Ignore the sequence of '\\' and
newline.
(read_element): If keys is an empty plist, return any element
found first.
(dump_plist_element): Check if plist is nested.
(mplist_conc): Always ref tail.
(mplist_set): If key is a managing key, ref val in advance.