Make zfs test easier to run in local install

When ZFS is installed by 'make install', programs will be installed
into '/usr/local'. ZFS test scripts can't locate programs 'zpool'
that caused tests failure.

Fix typo in help message.

Add sanity check to for ksh and generate a useful error message.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4495

scripts/zfs-tests.sh

@@ -177,7 +177,7 @@ $0 -r linux-fast
 
 # Cleanup a previous run of the test suite prior to testing, run the
 # default (linux) suite of tests and perform no cleanup on exit.
-$0 -c
+$0 -x
 
 EOF
 }
@@ -250,6 +250,13 @@ if [ $(sudo whoami) != "root" ]; then
 	fail "Passwordless sudo access required."
 fi
 
+#
+# Check if ksh exists
+#
+if [ -z "$(which ksh 2>/dev/null)" ]; then
+	fail "This test suite requires ksh."
+fi
+
 #
 # Verify the ZFS module stack if loaded.
 #