OpenZFS 9286 - want refreservation=auto

Authored by: Mike Gerdts <mike.gerdts@joyent.com>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Don Brady <don.brady@delphix.com>

Porting Notes:

• Adopted destroy_dataset in ZTS test cleanup
• Use ksh shebang instead of bash for new tests

OpenZFS-issue: https://www.illumos.org/issues/9286
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/723d0c85
Closes #7387