Add support for IPv6 and dual mode gprs contexts.
Gemalto ELS81x modems use 'ME PDN DEACT' message to notify about gprs context deactivation. Process this 'deactivate' event in CGEV handler.
Some gemalto modems provide USB ethernet interfaces for data path. Implement gprs-context driver for such modems to send data via USB ethernet rather than fallback to PPP.