zpool: Add slot power control, print power status

Add zpool flags to control the slot power to drives. This assumes
your SAS or NVMe enclosure supports slot power control via sysfs.

The new --power flag is added to zpool offline|online|clear:

zpool offline --power <pool> <device>    Turn off device slot power
zpool online --power <pool> <device>     Turn on device slot power
zpool clear --power <pool> [device]      Turn on device slot power

If the ZPOOL_AUTO_POWER_ON_SLOT env var is set, then the '--power'
option is automatically implied for zpool online and zpool clear
and does not need to be passed.

zpool status also gets a --power option to print the slot power status.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mart Frauenlob <AllKind@fastest.cc>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15662