Added Unix domain socket support

[ng88]

This adds support for Unix domain sockets (both datagrams and streams). It is mostly a hack, and if there is interest in this feature, it might be worthwhile to refactor the main code a bit. For instance, by introducing base classes for datagrams (to be derived for UDP and Unix datagrams) and streams (to be derived for TCP and Unix streams). Ideally, the ip_version parameter should also be renamed to something more generic, like family.

That said, this branch works as intended and remains fully backward compatible with previous versions.

[tglane]

Hey, I like the idea of adding unix domain socket support. But as you said, we should at least rename the ip_version type to something like family and than rename ip_version::v4 to family::ip_v4 and so on.

And than it might be enough to rename udp_socket class to datagram_socket and add using declarations for the ip versions and the unixsocket. That should not be to much work I think.

Would you be willing to add those changes or what do you think?

[ng88]

Yes indeed but it will break backward compatibility, is that a problem for you?

[tglane]

Not really. It's just a fun side project without any real world users. If I want to try out something new (like the coroutine stuff) I just play with it. So there is no real stable version or something like that.