[svn-r3121] Purpose:

Fix and improve

Description:
	- Put functions that are common to H5File and Group into a
	  prototype class, CommonFG.  I didn't do that before because
	  of the fear of the consequences of multiple inheritance, since
	  H5File and Group already inherit from different super classes.
	  I recently read a C++ book and learned to use MI more safely.
	  This change reduced some more of code redundancy.
	- Added missing const to some function parameters
	- Added missing return statements for some functions.

Platforms tested:
	Solaris/CC 5.0 (arabica)

c++/src/H5DataSet.cpp

@@ -102,7 +102,7 @@ hsize_t DataSet::getVlenBufSize( DataType& type, DataSpace& space ) const
    //{
       //throw DataSetIException();
    //}
-   return(-1);
+   return (-1);
 }
 
 // Reclaims VL datatype memory buffers.