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/Makefile.am

@@ -36,6 +36,7 @@ zpoolexecdir = $(zfsexecdir)/zpool.d
 EXTRA_DIST = zpool.d/README
 
 dist_zpoolexec_SCRIPTS = \
+	zpool.d/dm-deps \
 	zpool.d/enc \
 	zpool.d/encdev \
 	zpool.d/fault_led \
@@ -50,7 +51,6 @@ dist_zpoolexec_SCRIPTS = \
 	zpool.d/serial \
 	zpool.d/ses \
 	zpool.d/size \
-	zpool.d/slaves \
 	zpool.d/slot \
 	zpool.d/smart \
 	zpool.d/smartx \
@@ -80,6 +80,7 @@ dist_zpoolexec_SCRIPTS = \
 	zpool.d/test_ended
 
 zpoolconfdefaults = \
+	dm-deps \
 	enc \
 	encdev \
 	fault_led \
@@ -94,7 +95,6 @@ zpoolconfdefaults = \
 	serial \
 	ses \
 	size \
-	slaves \
 	slot \
 	smart \
 	smartx \