Support linux kernel < 3.9

[jr01]

rust-mdns uses SO_REUSEPORT option to bind to a port, that option isn't supported on kernels < 3.9. Many older devices can't be (easily) upgraded to a more recent kernel thereby leaving many not being able to use this library and spotify connect in librespot.

I suggested a fix here: https://github.com/plietar/librespot/issues/226#issuecomment-329964320

[LinusU]

Should be fixed by #24 :)