Search $PATH for the shutdown binary instead of hard-coding /sbin/shutdown,
which isn't valid on NixOS (and a compatibility link on most other modern
distros anyway).

  -- nckx <tobias.geerinckx.rice@gmail.com>

diff -ru util-linux-2.27-orig/include/pathnames.h util-linux-2.27/include/pathnames.h
--- util-linux-2.27-orig/include/pathnames.h	2015-06-29 13:13:14.669847478 +0200
+++ util-linux-2.27/include/pathnames.h	2015-10-07 20:09:17.401022602 +0200
@@ -54,7 +54,7 @@
 #define _PATH_INITTAB		"/etc/inittab"
 #define _PATH_RC		"/etc/rc"
 #define _PATH_REBOOT		"/sbin/reboot"
-#define _PATH_SHUTDOWN		"/sbin/shutdown"
+#define _PATH_SHUTDOWN		"shutdown"
 #define _PATH_SINGLE		"/etc/singleboot"
 #define _PATH_SHUTDOWN_CONF	"/etc/shutdown.conf"
 
diff -ru util-linux-2.27-orig/sys-utils/rtcwake.c util-linux-2.27/sys-utils/rtcwake.c
--- util-linux-2.27-orig/sys-utils/rtcwake.c	2015-08-05 11:32:44.453821232 +0200
+++ util-linux-2.27/sys-utils/rtcwake.c	2015-10-07 20:09:37.834032536 +0200
@@ -576,7 +576,7 @@
 		arg[i++] = "now";
 		arg[i]   = NULL;
 		if (!ctl.dryrun) {
-			execv(arg[0], arg);
+			execvp(arg[0], arg);
 			warn(_("failed to execute %s"), _PATH_SHUTDOWN);
 			rc = EXIT_FAILURE;
 		}