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

man/man8/zpool-clear.8

@@ -36,6 +36,7 @@
 .Sh SYNOPSIS
 .Nm zpool
 .Cm clear
+.Op Fl -power
 .Ar pool
 .Oo Ar device Oc Ns …
 .
@@ -52,6 +53,16 @@ Pools with
 enabled which have been suspended cannot be resumed.
 While the pool was suspended, it may have been imported on
 another host, and resuming I/O could result in pool damage.
+.Bl -tag -width Ds
+.It Fl -power
+Power on the devices's slot in the storage enclosure and wait for the device
+to show up before attempting to clear errors.
+This is done on all the devices specified.
+Alternatively, you can set the
+.Sy ZPOOL_AUTO_POWER_ON_SLOT
+environment variable to always enable this behavior.
+Note: This flag currently works on Linux only.
+.El
 .
 .Sh SEE ALSO
 .Xr zdb 8 ,