dev: add ability to pass --server-is-stopped to update.sh

This enables (semi-)automated upgrading:

```sh
sudo systemctl stop firefish.service
./update.sh --native --server-is-stopped
sudo systemctl start firefish.service
```

see also: d14bc4fb13

neko/update/native.sh

@@ -4,7 +4,7 @@ set -eu
 . neko/update/utils
 
 # Confirm that the server is stopped
-if ! contains '--install' "$@"; then
+if ! contains '--install' "$@" && ! contains '--server-is-stopped' "$@"; then
   say 'Did you stop your server?'
   printf '[Y/n] > '
   read -r yn