Remove unnecessary references to slavery

The horrible effects of human slavery continue to impact society.  The
casual use of the term "slave" in computer software is an unnecessary
reference to a painful human experience.

This commit removes all possible references to the term "slave".

Implementation notes:

The zpool.d/slaves script is renamed to dm-deps, which uses the same
terminology as `dmsetup deps`.

References to the `/sys/class/block/$dev/slaves` directory remain.  This
directory name is determined by the Linux kernel.  Although
`dmsetup deps` provides the same information, it unfortunately requires
elevated privileges, whereas the `/sys/...` directory is world-readable.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10435

cmd/zpool/zpool.d/dm-deps

@@ -0,0 +1,29 @@
+#!/bin/sh
+#
+# Show device mapper dependent / underlying devices.  This is useful for
+# looking up the /dev/sd* devices associated with a dm or multipath device. 
+#
+
+if [ "$1" = "-h" ] ; then
+	echo "Show device mapper dependent (underlying) devices."
+	exit
+fi
+
+dev="$VDEV_PATH"
+
+# If the VDEV path is a symlink, resolve it to a real device
+if [ -L "$dev" ] ; then
+	dev=$(readlink "$dev")
+fi
+
+dev=$(basename "$dev")
+val=""
+if [ -d "/sys/class/block/$dev/slaves" ] ; then
+	# ls -C: output in columns, no newlines
+	val=$(ls -C "/sys/class/block/$dev/slaves")
+
+	# ls -C will print two spaces between files; change to one space.
+	val=$(echo "$val" | sed -r 's/[[:blank:]]+/ /g')
+fi
+
+echo "dm-deps=$val"