Do not start npfctl again when started via npf_boot.

Index: npf
===================================================================
RCS file: /cvsroot/src/etc/rc.d/npf,v
retrieving revision 1.3
diff -u -p -r1.3 npf
--- npf	1 Nov 2012 06:06:14 -0000	1.3
+++ npf	19 Apr 2019 17:11:44 -0000
@@ -36,7 +36,11 @@ npf_start()
 	echo "Enabling NPF."
 	npf_cfg_check
 	/sbin/npfctl reload
-	/sbin/npfctl start
+
+	# The npf_boot script has enabled npf already.
+	if [ "$autoboot" != "yes" ]; then
+		/sbin/npfctl start
+	fi
 }
 
 npf_stop()