start: Add s6 startup script

The `start` script launches `s6-svscan` as a daemon. It is designed to
be run by cron, e.g.

    @runatreboot,runonce 1 ~/.s6/start

start

@@ -0,0 +1,2 @@
+#!/bin/sh
+nohup /bin/s6-svscan -t 0 ~/.s6/scan >> ~/.s6/log 2>&1 &