Further improve previous revision:

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

git-svn-id: svn://svn.freebsd.org/base/stable/8@249114 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f