[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/H5IdComponent.cpp

@@ -82,7 +82,7 @@ IdComponent& IdComponent::operator=( const IdComponent& rhs )
         its identifier, its reference counter will be deleted.  A new 
         reference counter is created for the new HDF5 object id.
 */
-void IdComponent::setId( const hid_t new_id )
+void IdComponent::setId( hid_t new_id )
 {
    // reset the identifier of this object, call appropriate H5Xclose
    resetIdComponent( this );