um: Added support for unix socket transports and bess transport

This adds support for the UNIX domain socket transports in
general and implements a Netsys::BESS compatible transport
interface.
For details on Netsys::BESS see https://github.com/NetSys/bess



Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>