Per Matthew Ahrens, version 5000 should not be exposed to user and there is
a problem with my first revision, namely, specifying -d -o feature@...=enable
will still bail out with:

'feature@' and 'version' properties cannot be specified together.

Because zpool create -o version=5000 will not likely be supported by other
ZFS implementations (including ours on -CURRENT and 9-STABLE), remove the
hack that make that work. Users who want feature flags support can still
do an explicit 'zpool upgrade' after creating a pool.