Fix detection of /usr/lib on NFS.

Found by Frank Scheiner, thanks for reporting this.
OK krw, halex
'cool' deraadt

src/etc/rc

@@ -1,4 +1,4 @@
-#	$OpenBSD: rc,v 1.485 2016/05/29 15:36:06 rpe Exp $
+#	$OpenBSD: rc,v 1.486 2016/07/10 09:08:18 rpe Exp $
 
 # System startup script run by init on autoboot or after single-user.
 # Output and error are redirected to console by init, and the console is the
@@ -161,7 +161,7 @@ make_keys() {
 # Re-link libraries, placing the objects in a random order.
 reorder_libs() {
 	local _l _liba _libas _tmpdir _remount=false _error=false
-	local _dkdev=/dev/$(stat -L -f '%Sd' /usr/lib)
+	local _dkdev=$(df /usr/lib | sed '1d;s/ .*//')
 	local _mp=$(mount | grep "^$_dkdev")
 
 	# Skip if /usr/lib is on a nfs mounted filesystem.