IP_STATUS_FILE and routing improvements (#130)

- Obtains VPN public IP address from routing table
- Logs and writes VPN Public IP address to `/ip` as soon as VPN is up
- Obtain port forward, logs it and writes it as soon as VPN is up
- Routing fully refactored and tested
- Routing reads from `/proc/net/route`
- Routing mutates the routes using `ip route ...`

Dockerfile

@@ -38,6 +38,7 @@ ENV VPNSP=pia \
     TZ= \
     UID=1000 \
     GID=1000 \
+    IP_STATUS_FILE="/ip" \
     # PIA only
     PASSWORD= \
     REGION="Austria" \