Add ConnectUnix and DialUnix

[anonymix007]

These allow creating D-Bus connections from *net.UnixConn on Unix systems.

File descriptors may be wrapped with

func fdToUnixConn(fd dbus.UnixFD, name string) (*net.UnixConn, error) {
	c, err := net.FileConn(os.NewFile(uintptr(fd), name))
	if err != nil {
		return nil, err
	}
    return c.(*net.UnixConn), nil
}

and will work as well.

This also technically introduces some sort of peer-to-peer support if only auth client is needed (i.e. QEMU).

Fixes #76
Partially fixes #384