Quick fix: exit after a failed execvp in u_spawn instead; previously we did in

u_exec, but the introduction of re-exec'ing the previous invocation of cwm if
'exec_wm' failed missed the 'exec' failing path. Will likely split out as a
proper fix.

Odd behaviour reported by Ve Telko.

util.c

@@ -37,7 +37,7 @@ u_spawn(char *argstr)
 	switch (fork()) {
 	case 0:
 		u_exec(argstr);
-		break;
+		exit(1);
 	case -1:
 		warn("fork");
 	default: