ZTS: Misc fixes for FreeBSD

ZTS: Misc fixes for FreeBSD

* Force UFS sync before snap in vol rollback tests
* rw is not a valid share option on FreeBSD, use ro instead
* zfs_unmount_nested: mountpoint is in the pool, rmdir *before* export
* Fix some more platform checks
* Fix disappearing group in delegate tests
* Don't try delegating for jailed, only root can set it

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10038