vdata_id	IN:	Vdata identifier returned by VSattach
	field_index	IN:	Vdata field index

	Purpose	Returns the name of a vdata field.
	Return value	Returns a pointer to the vdata field name if successful and NULL otherwise. The FORTRAN-77 version of this routine, vffname, returns SUCCEED (or 0) or FAIL (or -1).
	Description	VFfieldname returns the name of the vdata field identified by the parameter field_index in the vdata identified by the parameter vdata_id.
		The value of the parameter field_index ranges from 0 to the total number of fields in the vdata - 1. The number of vdata fields is returned by VFnfields function.
		The FORTRAN-77 version of this routine, vffname, returns the field name in the parameter fname.

	FORTRAN	integer function vffname(vdata_id, field_index, fname)
		integer vdata_id, field_index
		character*(*) fname