OpenZFS 4521 - zfstest is trying to execute evil "zfs unmount -a"

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

Porting Notes:
- Correctly set __ZFS_POOL_RESTRICT in inherit_001_pos

OpenZFS-issue: https://www.illumos.org/issues/4521
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8808ac5
Closes #5674

scripts/zfs-tests.sh

@@ -279,6 +279,8 @@ if [ -z "${KEEP}" ]; then
 	fi
 fi
 
+__ZFS_POOL_EXCLUDE="$(echo $KEEP | sed ':a;N;s/\n/ /g;ba')"
+
 msg
 msg "--- Configuration ---"
 msg "Runfile:         $RUNFILE"
@@ -341,6 +343,7 @@ export STF_TOOLS
 export STF_SUITE
 export DISKS
 export KEEP
+export __ZFS_POOL_EXCLUDE
 
 msg "${TEST_RUNNER} ${QUIET} -c ${RUNFILE} -i ${STF_SUITE}"
 ${TEST_RUNNER} ${QUIET} -c ${RUNFILE} -i ${STF_SUITE}