Implementation of router alert handling

This set of commits implements the handling of router alert headers for IPv4 and IPv6. It also fixes a bug in IPvXTrafGen which prevented it from working in a non-flat IPv6 network. The router alert handling implementation allows protocol developers to intercept packets at each intermediate node in a network e.g. to implement new QoS signalling protocols, while still being able to reuse the standard Router and Host modules. The implementation is somewhat preliminary in a sense, that it does not fix the issues with the option header handling in IPv4. It still uses only one option header there.