GUnixSocketAddress: handle abstract sockets with non-0-padded names

There are apparently two incompatible ways of naming abstract sockets:
pad the sockaddr with 0s and use the entire thing as the name, or else
don't, and just pass a shorter length value to the relevant functions.
We previously only supported the former method. Add support for the
latter.

Also correctly handle "anonymous" unix sockaddrs (eg, the client side
of a connection, or a socketpair() socket), and add unix domain socket
support to the socket-client and socket-server test programs to make
sure this all works.

https://bugzilla.gnome.org/show_bug.cgi?id=615960