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

include/os/freebsd/spl/sys/dkio.h

@@ -69,7 +69,6 @@ struct dk_cinfo {
 	uint_t	dki_vec;		/* interrupt vector */
 	char	dki_dname[DK_DEVLEN];	/* drive name (no unit #) */
 	uint_t	dki_unit;		/* unit number */
-	uint_t	dki_slave;		/* slave number */
 	ushort_t dki_partition;		/* partition number */
 	ushort_t dki_maxtransfer;	/* max. transfer size in DEV_BSIZE */
 };