Experimental: Fix tethering
1. ARCHIDROID_CRON is obsolete
2. Improve internal communication between ArchiDroid Backend, especially tethering
3. Tethering now informs ARCHIDROID_INIT about the event directly
4. ARCHIDROID_MONITOR_DNS is now part of ARCHIDROID_INIT, previously: ARCHIDROID_CRON
5. Misc improvements here and there