zdb.8: replace with the slighly modified upstream version
The upstream has converted their manual page to the same format as we
have, so we can use the upstream version with minimal modifications.
The current modifications are:
- different zpool.cache path
- different manual sections for zdb, zfs, zpool commands
igor reports a few minor issues, it would be nice to fix them both in
FreeBSD and in the upstream.
MFC after: 3 weeks