camcontrol: depop command

Implement and document the new depop command. This command manages drive elements
for drives that support it. Storage elements are typically heads. Element status
can be discovered. Elements may be removed or restored. And the status of any
current depop operation can be assessed.

depop -d elm will remove element elm and truncate available capacity.
depop -l will list the current drive elements and their current status.
depop -r elm will try to restore all retired elements and rebuild capacity.

Changing storage elements may reinitialize the drive. This operation will lose
data and may take hours to complete. Use the drive provided timeout for
operations by default.

Reviewed by: gbe (manpages)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29018