Support path_id changes in udev 174.

The /lib/udev/path_id helper became a builtin command in the udev 174
release, so test whether path_id is external in the zpool_id script.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes: #429

cmd/zpool_id/zpool_id

@@ -1,9 +1,17 @@
 #!/bin/bash
 
 CONFIG=${CONFIG:-/etc/zfs/zdev.conf}
-PATH_ID=${PATH_ID:-/lib/udev/path_id}
 AWK=${AWK:-/usr/bin/awk}
 
+if [ -z "${PATH_ID}" ]; then
+	# The path_id helper became a builtin command in udev 174.
+	if [ -x '/lib/udev/path_id' ]; then
+		PATH_ID='/lib/udev/path_id'
+	else
+		PATH_ID='udevadm test-builtin path_id'
+	fi
+fi
+
 die() {
 	echo "Error: $*"
 	exit 1