Apply modified version of patch from Wolfgang Stukenbrock in PR bin/41766

This forcibly unmounts tmpfs file systems, so that swapctl doesn't run
out of swap itself, eventually trying to kill pid 1.

No objections since:
  http://mail-index.netbsd.org/current-users/2015/03/16/msg026908.html

etc/rc.d/swap1

@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# $NetBSD: swap1,v 1.11 2009/09/24 16:45:48 apb Exp $
+# $NetBSD: swap1,v 1.12 2015/04/20 18:01:46 prlw1 Exp $
 #
 
 # PROVIDE: localswap
@@ -31,6 +31,8 @@ swap1_start()
 swap1_stop()
 {
 	if checkyesno swapoff || [ -n "$rc_force" ]; then
+		echo "Forcibly unmounting tmpfs filesystems"
+		umount -aft tmpfs
 		echo "Removing block-type swap devices"
 		swapctl -U -t blk || [ $? = 2 ]
 	fi