pf: Friendly error message for status if pf.ko is not loaded

Check if pf.ko is loaded (i.e. /dev/pf exists) before trying to use it. This
means that '/etc/rc.d/pf status' will no longer return 'pfctl: /dev/pf: No such
file or directory' but 'pf.ko is not loaded'.

PR:		205671
Submitted by:	Johannes Jost Meixner <xmj@FreeBSD.org>

etc/rc.d/pf

@@ -66,7 +66,11 @@ pf_resync()
 
 pf_status()
 {
-	$pf_program -s info
+	if ! [ -c /dev/pf ] ; then
+		echo "pf.ko is not loaded"
+	else
+		$pf_program -s info
+	fi
 }
 
 run_rc_command "$1"