python: only set SO_REUSEPORT on BSD

On Linux SO_REUSEPORT does load balancing (see https://stackoverflow.com/a/14388707), which is not what we want. So if we are not on Linux and SO_REUSEPORT is available, we are on BSD and need to set it. Windows doesn't have it so the existing code would result in a crash on Windows. In C++, asio sets SO_REUSEPORT automatically on BSD if SO_REUSEADDR is set.

Tested on Linux and it (still) works. I haven't tried out if we can actually get CHIRP packages to be dropped due to the load balancing, but in any case we don't want to have it.