Cosmetics: rename ff_parse_expr() and ff_parse_and_eval_expr() parameters:

const_name    -> const_names
const_value   -> const_values
func[12]_name -> func[12]_names
func[12]      -> funcs[12]

All these parameters contain a list of values, using plural names for
them help understanding a little.

Originally committed as revision 23403 to svn://svn.ffmpeg.org/ffmpeg/trunk