env: Check unsetenv() return value

./env -u "" foo and ./env -u "A=123" foo should both fail.

env.c

@@ -25,7 +25,8 @@ main(int argc, char *argv[])
 		*environ = NULL;
 		break;
 	case 'u':
-		unsetenv(EARGF(usage()));
+		if (unsetenv(EARGF(usage())) < 0)
+			eprintf("unsetenv:");
 		break;
 	default:
 		usage();