Ticket 574 - problems with dbcachesize disk space calculation
    
    Bug Description: no_diskpace() in dblayer.c was not calculating the available
                     diskspace correctly.  It was checking the main db directory
                     only, and it never cheked for db_home_directory.
    
    Fix Description: Check for db_home_directory first, and if it's shared/private
                     memory then skip the check altogether.  Otherwise calulate the
                     correct directory size, and also add to the available space the
                     size of the region files(__db* files) - as these should be
                     ignored.
    
                     Also did a little code cleanup of the header files.
    
    https://fedorahosted.org/389/ticket/574
    
    Reviewed by: Noriko(Thanks!)