Endianness: make sure that it runs OK on non-little-endian machines,
such as 68k, powerpc.


Find out about the btree_dir feature.  I suspect this feature can be
supported with zero code change (other than adding
EXT2_FEATURE_RO_COMPAT_BTREE_DIR to set of supported ro_features).
However, current mainstream kernels (2.4.0) and e2fsprogs (1.19) don't
really use this feature, so it's hard to be sure what's needed.


See also tests/TODO.
