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