hdf5_msb

Add `-a steps` option and if steps != 0, then add (or verify) an attribute on
each dataset every `steps` steps.

Update usage message.

Add a cast to `time_t` to quiet a compiler warning.

Replace two occurrences of a debug statement in `verify_extensible_dset()` with
one occurrence in `verify_chunk()`.

Replace the anonymous constant `2` with `hang_back` and increase `hang_back` to
3.  XXX Now that I've fixed a bug, reduce `hang_back` to 2, again.

Verify datasets in the reverse of the order they are written so that we spend
less time re-verifying datasets written in the same step.