Change value in PLIST of PROP to VAL.
PLIST is a property list, which is a list of the form
(PROP1 VALUE1 PROP2 VALUE2 ...). PROP is a symbol and VAL is any object.
If PROP is already a property on the list, its value is set to VAL,
otherwise the new PROP VAL pair is added. The new plist is returned;
use `(setq x (plist-put x prop val))' to be sure to use the new value.
The PLIST is modified by side effects. (fn PLIST PROP VAL)