Turn on/off enclosure slot fault LED even when disk isn't present

Previously when a drive faulted, the statechange-led.sh script would lookup
the drive's LED sysfs entry in /sys/block/sd*/device/enclosure_device, and
turn it on.  During testing we noticed that if you pulled out a drive, or if
the drive was so badly broken that it no longer appeared to Linux, that the
/sys/block/sd* path would be removed, and the script could not lookup the
LED entry.

To fix this, this patch looks up the disks's more persistent
"/sys/class/enclosure/X:X:X:X/Slot N" LED sysfs path at pool import.  It then
passes that path to the statechange-led script to use, rather than having the
script look it up on the fly.  This allows the script to turn on/off the slot
LEDs even when the drive is missing.

Closes #5309 
Closes #2375

cmd/zed/zed_disk_event.c

@@ -286,7 +286,7 @@ zed_udev_monitor(void *arg)
 		    udev_device_get_property_value(dev, "DM_UUID") &&
 		    udev_device_get_property_value(dev, "MPATH_SBIN_PATH")) {
 			tmp = (char *) udev_device_get_devnode(dev);
-			tmp2 = get_underlying_path(NULL, tmp);
+			tmp2 = zfs_get_underlying_path(tmp);
 			if (tmp && tmp2 && (strcmp(tmp, tmp2) != 0)) {
 				/*
 				 * We have a real underlying device, which