ipc-unix: don't blow up on readonly fd://

On my system, when trying to use mpv with a read-only fd created by
python 3.7, `send` triggers ENOTSOCK, not EBADF. Extend the logic to
detect non-writable fds by this `errno`.

input/ipc-unix.c

@@ -73,7 +73,7 @@ static int ipc_write_str(struct client_arg *client, const char *buf)
             if (rc == 0)
                 return -1;
 
-            if (errno == EBADF) {
+            if (errno == EBADF || errno == ENOTSOCK) {
                 client->writable = false;
                 return 0;
             }