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

[guelfey]

This regresses #223 , see e.g. https://github.com/godbus/dbus/actions/runs/19359654286/job/55389087192?pr=399#step:6:255. Why did you include this change?

[anonymix007]

Because it was crashing otherwise. I don't remember the full details, but here's the code that I was using this PR with: https://github.com/anonymix007/qemu-godisplay
AFAIR it was panicking somewhere, but because of recover it was just silently returning an empty fd array in ScanoutDMABUF2

[guelfey]

Ok, this would be an unrelated change then; and even then, would need to be solved in a different way. I will cherry-pick the first 2 commits that add ConnectUnix and DialUnix. If the FD decoding behavior is still important for you, please create a separate issue with more details.

[anonymix007]

I switched to another language for that project since Go just doesn't work well with GStreamer, so it's not really important anymore.

[guelfey]

merged as #417