MFC r312118,r312121:

r312118:

Fix -Wformat issue with zero-length format string passed to err(3)

Tested with:	clang, gcc 4.2.1, gcc 4.9

r312121:

Follow up to r312118

State that execve failed instead of just printing out the program name
and strerror(errno) via err(3).

tests/sys/kern/execve/execve_helper.c

@@ -50,5 +50,5 @@ main(int argc, char **argv)
 	}
 
 	execve(argv[1], &argv[1], NULL);
-	err(1, "");
+	err(1, "execve failed");
 }