hdf5_der

[svn-r23231] Changes for backward/forward compatibility:
1) Enable "mark if unknown" flag when creating the file space info message
2) Use default file space management if the file space info message is marked as "unknown"
3) Generate test files that are copied to 1.8 branch, opened/modified by 1.8 library,
and then copied back to the trunk for compatibility test via test_filespace_round_compatible().