Illumos #2635: 'zfs rename -f' to perform force unmount

Illumos #2635: 'zfs rename -f' to perform force unmount

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: George Wilson <George.Wilson@delphix.com>
Reviewed by: Bill Pijewski <wdp@joyent.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  https://www.illumos.org/issues/2635

Ported by: Martin Matuska <martin@matuska.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #717