Use event loop instead of threads

[arnout]

Threads are annoying to work with, dangerous, more difficult to port, and not resource friendly. Also, on some platforms, it's easier to implement the behaviour as callbacks (on packet receive, possibly from interrupt) than as a thread.

The only reason to use threads is to be able to distribute work over several processors. But IEEE1905.1 doesn't have much work, just some packet processing.

Therefore, we'd prefer to use an event loop.

We can use an existing library, e.g. libev.