Stop the HDF5 library installing an exit hook

This will cause XDS to segfault on exit, since the plugin (and hence
HDF5 library) will likely have been unloaded.

Calling "H5dont_atexit" before any other H5 calls will prevent the
hook being installed, and we call "H5Close" ourselves as part of the
plugin_close function.

test/generic_data_plugin.f90

@@ -431,10 +431,12 @@ contains
     end if
 
     write(6,*) "[generic_data_plugin] - INFO - 'call generic_close()'"
-    
+    external_error_flag=0
     call dll_plugin_close(external_error_flag)
     error_flag = external_error_flag
 
+    write(6,*) "[generic_data_plugin] - INFO - 'plugin close flag:", error_flag, "'"
+
     ! now close the dl:
     status=dlclose(handle)
     if(status/=0) then