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.