Further improve previous revision:

• enable_all_pool_feat will be unset if version is specified. Use it as a flag instead of testing the props nvlist;
• Allow user to use -d -o feature@...=enable to create a v5000 pool when desired. Without this, the implicit -o version=28 would make the utility to complain about feature@ and version being conflict, which is confusing.

Reviewed by: Matthew Ahrens