The current ZFS version in 8-STABLE supports feature flags, which
enables many new features but makes it impossible to import pools
created on earlier released FreeBSD 9.x releases, including 9.0
and 9.1-RELEASE, where the feature flags are not yet supported
because they predates the merge (r243674), and 9.2-RELEASE will
not be released before 8.4-RELEASE.

To avoid surprises when users "upgrade" to 9.1-RELEASE, limit the
creation version to 28 by default on stable/8. The user will still
be able to upgrade the pool by using "zpool upgrade" or at create
time by explicitly specifying "zpool create -o version=5000".

This is a direct commit to stable/8 because it's not applicable to
-HEAD, and can be reverted once 9.2-RELEASE is released.

Requested by: re (jpaetzel, hrs)
Reviewed by: mm