Mesh IPs in header are discovered in an OLSR-specific way, and don't work for all use cases

[jheretic]

Pull request #413 works but introduces a separate conditional for parsing the OLSRd configuration file to look for mesh-over-ethernet interfaces. This doesn't cover the use case of multiple wireless meshed interfaces, and can probably be merged with the original conditional by simply looking for interfaces with the commotion protocol that have the 'meshed' UCI state variable.

Given that there's a limit to how many IPs we can fit in the header anyway, this should probably eventually be replaced by different addressing techniques that mean there is only one mesh IP for all interfaces, but that's further in the future.