Don't kill unionfs-fuse during shutdown

Killing the daemon backing /nix/store prevents a clean shutdown.  See
http://www.freedesktop.org/wiki/Software/systemd/RootStorageDaemons/

pkgs/tools/filesystems/unionfs-fuse/default.nix

@@ -8,6 +8,14 @@ stdenv.mkDerivation rec {
     sha256 = "0qpnr4czgc62vsfnmv933w62nq3xwcbnvqch72qakfgca75rsp4d";
   };
 
+  patches =
+    [ # Prevent the unionfs daemon from being killed during
+      # shutdown. See
+      # http://www.freedesktop.org/wiki/Software/systemd/RootStorageDaemons/
+      # for details.
+      ./prevent-kill-on-shutdown.patch
+    ];
+
   buildInputs = [ cmake fuse ];
 
   # Put the unionfs mount helper in place as mount.unionfs-fuse. This makes it

pkgs/tools/filesystems/unionfs-fuse/prevent-kill-on-shutdown.patch

@@ -0,0 +1,14 @@
+diff -ru -x '*~' unionfs-fuse-0.26-orig/src/unionfs.c unionfs-fuse-0.26/src/unionfs.c
+--- unionfs-fuse-0.26-orig/src/unionfs.c	2012-09-11 00:06:32.000000000 +0200
++++ unionfs-fuse-0.26/src/unionfs.c	2014-05-21 14:22:03.597861805 +0200
+@@ -831,6 +831,10 @@
+ 	}
+ #endif
+ 
++	/* Prevent systemd's shutdown from killing us. */
++	if (access("/etc/initrd-release", F_OK) >= 0)
++		argv[0][0] = '@';
++
+ 	umask(0);
+ 	int res = fuse_main(args.argc, args.argv, &unionfs_oper, NULL);
+ 	RETURN(uopt.doexit ? uopt.retval : res);