Return non-nil if PLIST has the property PROP.
PLIST is a property list, which is a list of the form
(PROP1 VALUE1 PROP2 VALUE2 ...). PROP is a symbol.
Unlike `plist-get', this allows you to distinguish between a missing
property and a property with the value nil.
The value is actually the tail of PLIST whose car is PROP. (fn PLIST PROP)