various: clean up isatty() handling

As per https://github.com/systemd/systemd/pull/30547#discussion_r1434371627

src/shared/wall.c

@@ -30,8 +30,9 @@ static int write_to_terminal(const char *tty, const char *message) {
         fd = open(tty, O_WRONLY|O_NONBLOCK|O_NOCTTY|O_CLOEXEC);
         if (fd < 0)
                 return -errno;
-        if (!isatty(fd))
-                return -ENOTTY;
+
+        if (!isatty_safe(fd))
+                return -errno;
 
         return loop_write_full(fd, message, SIZE_MAX, TIMEOUT_USEC);
 }