terminal-util: don't assume errno is correctly set when using isatty_safe()

let's instead generate ENOTTY on our own. This is more correct with out
coding style (since we generally do not propagate errors via errno), and
also addresses #34039 as side effect. (#34039 really needs to be fixed
in musl though, too, this is just a work-around as side-effect).

Fixes: #34039

src/getty-generator/getty-generator.c

@@ -95,7 +95,7 @@ static int verify_tty(const char *name) {
                 return -errno;
 
         if (!isatty_safe(fd))
-                return -errno;
+                return -ENOTTY;
 
         return 0;
 }