hdf5_ep

[svn-r21870] Purpose:
  Fix for HDFFV-7840 h5repack: memory leak over one of the h5diff test file

Description:
  Turned out that there were two causes of memory leaks.
  1. for handling variable length string in attribute. 
  2. for handling compound type with non-reference members.  
  The first issue is fixed in copy_attr() which is updated to use h5tools_detect_vlen to take care of vlen string as well as vlen data. 
  The second is fixed in copy_refs_attr() of compound handling code.
  Merged from HDF5 trunk r21869.
  
Tested:
 jam (linux32-LE), koala (linux64-LE), heiwa (linuxppc64-BE), tejeda (mac32-LE), linew (solaris-BE),  Windows (32-LE cmake), Cmake (jam)