Illumos #510: 'zfs get' enhancement - mountpoint as an argument

Illumos #510: 'zfs get' enhancement - mountpoint as an argument

The 'zfs get' command should be able to deal with mountpoint
as an argument.  It already works with 'zfs list' command:

  # zfs list /export/home/estibi
  NAME                       USED  AVAIL  REFER  MOUNTPOINT
  rpool/export/home/estibi  1.14G  3.86G  1.14G  /export/home/estibi

but it fails with 'zfs get':

  # zfs get all /export/home/estibi
  cannot open '/export/home/estibi': invalid dataset name

Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Deano <deano@rattie.demon.co.uk>
Reviewed by: Garrett D'Amore <garrett@nexenta.com>
Approved by: Garrett D'Amore <garrett@nexenta.com>

References to Illumos issue and patch:
- https://www.illumos.org/issues/510
- https://github.com/illumos/illumos-gate/commit/5ead3ed965

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #340