Destintion and Sender are optional fields, handle that

This is not typically the case for bus-uses, but is common when
using custom peer-to-peer connections.
Add support for acting as a dbus server

This lets you listen on a dbus address for clients connecting.
Add peer-to-peer example

This is a server/client pair that shows how to use peer-to-peer
connections. You can also verify the server with gdbus like:

 gdbus call -a unix:path=./socket -o /com/github/guelfey/Demo/PeerServer -m com.github.guelfey.PeerServer.Foo 42

 # Please enter the commit message for your changes. Lines starting
Always enable unix FDs for unix servers

Normally this is enabled after negotiation on the client side, but
we know it is supported in the server.
Without this we can't return Unix FDs on server connections.
Move ServerAuth out of Server()

We want to do auth after we've correctly set up the connection (by
e.g. exporting objects), otherwise there can be races where the
client starts sending requests before we've yet exported some
object.
Merge remote-tracking branch 'alexlarsson/peer-to-peer'