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

tests/zfs-tests/tests/functional/xattr/setup.ksh

@@ -32,9 +32,7 @@
 
 # if we're running NIS, turn it off until we clean up
 # (it can cause useradd to take a long time, hitting our TIMEOUT)
-if is_linux; then
-	USED_NIS=false
-else
+if is_illumos; then
 	USES_NIS=false
 	svcs svc:/network/nis/client:default | grep online > /dev/null
 	if [ $? -eq 0 ]
@@ -42,6 +40,8 @@ else
 		svcadm disable -t svc:/network/nis/client:default
 		USES_NIS=true
 	fi
+else
+	USES_NIS=false
 fi
 
 # Make sure we use a brand new user for this