Bug fixes
Reset external file list slots name_offset to a state when created (0) in H5P_dcrt_copy
so that it conforms to an assertion in H5D_update_entry_info that assumes the name_offset is 0 at this point
this fixes the problem of h5repack and external files, add a new test and files for an external file
h5diff, check for an error return in H5D_get_storage_size
tested linux 32, 64
