hdf5

[svn-r23907] - add a new routine:
	herr_t H5VLget_object(hid_t obj_id, void **obj, H5VL_t **vol_plugin)
  to retrieve object pointer from given ID. This also optionally returns a pointer to a vol_plugin if it is not NULL.

- remove the previously added routine to retrieve the VOL datatype object since the new one does that.

- add a MAX_LIB_VOL_VALUE that reserves 128 value for internal VOL plugins, not that we need that many.