Implement Unix file descriptor passing

[guelfey]

Neither the authentication, nor the marshalling code currently support the passing of unix file descriptors.

[guelfey]

Plan of attack:

1. Add support for the Unix file descriptor type in proto.go and message.go, creating type UnixFD int32.
2. Define a transport interface that is used instead of a net.Conn, which will also help to fix Support transports besides unix #6. This interface then defines methods like SendMessage and ReadMessage, which could handle things like file descriptor passing. Also, the method SupportsUnixFDs (returning a bool) should be defined.
3. Implement this interface for the unix transport, including file descriptor passing.
4. Modify Connection to work with this new interface:
   • Send a NEGOTIATE_UNIX_FD in the auth protocol if the transport supports it.
   • Track whether unix fds are supported for the connection (new struct member). (Add a method that returns this value?)
   • Reject messages containing Unix fds if they're not supported.

[guelfey]

Fixed by 299651a.