[syncthing] Fix #340

[salim-b]

Beware that this PR is untested.

I'm unsure whether it's fine to do it like this. Maybe the main syncthing invocation (syncthing --gui-address="$ip:8384" --no-browser) must come last in the S6 run script for HA to be able to detect when the add-on crashes (and automatically restart it if desired)? In that case I wouldn't know how to run syncthing cli config gui insecure-admin-access set true since syncthing cli commands only work when Syncthing is already running...