illumos gate

At the moment type parameter of lzc_create() is of dmu_objset_type_t type.
That exposes an implementation detail and requires sys/fs/zfs.h to be included in libzfs_core.h creating unnecessary coupling between libzfs_core interface and ZFS internals.
I think that dmu_objset_type_t should be replaced with a libzfs_core enumeration of supported dataset types.
For ABI reasons the new enumeration could be bit-compatible with dmu_objset_type_t.
For example:

    typedef enum {
        LZC_DST_ZFS = 2,
        LZC_DST_ZVOL
    } lzc_dataset_type_t;