Fix syncthing process reparenting with runit

When you: `sudo sv down /etc/service/syncthing/` the `TERM` signal
isn't propogated or trapped, so syncthing is orphaned and adopted by
init (PID 1).

- Changed call to `chpst` to `exec`
- Moved logging to `log/run` per `runsv` standard

etc/linux-runit/log/run

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+exec logger -t syncthing
+