MFV r298471: 6052 decouple lzc_create() from the implementation details

MFV r298471: 6052 decouple lzc_create() from the implementation details

illumos/illumos-gate@26455f9efcf9b1e44937d4d86d1ce37b006f25a9
https://github.com/illumos/illumos-gate/commit/26455f9efcf9b1e44937d4d86d1ce37b006f25a9

https://www.illumos.org/issues/6052
  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;

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Andriy Gapon <andriy.gapon@clusterhq.com>

MFC after:	2 weeks
Sponsored by:	ClusterHQ