Window functions: Frame Exclusion support

opt_window_frame_clause:

          /* empty */ {}

        | window_frame_units window_frame_extent opt_window_frame_exclusion

opt_window_frame_exclusion:

          /* empty */ { $$= Window_frame::EXCL_NONE; }

        | EXCLUDE_SYM CURRENT_SYM ROW_SYM

          { $$= Window_frame::EXCL_CURRENT_ROW; }

        | EXCLUDE_SYM GROUP_SYM

          { $$= Window_frame::EXCL_GROUP; }

        | EXCLUDE_SYM TIES_SYM

          { $$= Window_frame::EXCL_TIES; }

        | EXCLUDE_SYM NO_SYM OTHERS_SYM

          { $$= Window_frame::EXCL_NONE; }

        ;