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

@@ -1,14 +1,11 @@
 #!/bin/sh
 #
-# Show device mapper slave devices.  This is useful for looking up the
-# /dev/sd* devices associated with a dm or multipath device.  For example:
-#
-# $ ls /sys/block/dm-113/slaves/
-# sddt  sdjw
+# 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 slave devices."
+	echo "Show device mapper dependent (underlying) devices."
 	exit
 fi
 
@@ -29,4 +26,4 @@ if [ -d "/sys/class/block/$dev/slaves" ] ; then
 	val=$(echo "$val" | sed -r 's/[[:blank:]]+/ /g')
 fi
 
-echo "slaves=$val"
+echo "dm-deps=$val"