Use virtual functions for implementation diversity

[arnout]

Currently, implementation diversity (e.g. how to get statistics from an interface) is governed by a combination of ifdefs and ad-hoc conditions. This makes it more difficult to support multiple implementations in a single system. For example, if a system uses Wifi chips from different vendors that use vendor-specific extensions rather than standard nl80211/cfg80211, we need a different interface implementation for each interface. However a large part of the different implementations can probably be shared.

Therefore, it is more convenient to use virtual functions, i.e. function pointers in the per-interface struct. This way, specific interface implementations only need to override the functions they care about.