Allow creating connections from *net.UnixConn

[mildred]

It may be that the connection was opened outside of libdbus (for example when inheriting from fork of an already opened file descriptor). The dbus library should allow to create a connection (with ability to pass fd) from an existing *net.UnixConn

This could be used to communicate using dbus between different processes that don't want to expose their communication to a system or user/session bus.

[jsouthworth]

Sounds like a good addition. Are you working on this or just requesting that it get done?

[mildred]

I am not working on this for the moment. I needed an IPC for two processes communicating directly together without a bus, and it doesn't seem easy to make it possible so I'll use a home-made IPC protocol instead.