add go-zfs dependency to hack/vendor.sh

If patches needed by docker zfs driver will be accepted mistifyio/go-zfs will
used instead of Mic92/go-zfs. Then the source will be also checked in dockers
version control

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: improve error message, when dataset is not readable

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: fix cases where lookupZfsDataset fails:

- don't abort if an unaccessible mount point is found
- check if mountpoint is really a zfs dataset

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: cleanup after image building was aborted

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: upgrade zfs-go to CamelCase constant version

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>