libknet: Add LOOPBACK transport

KNET_TRANSPORT_LOOPBACK is a transport designed for
high-speed communication internal to the local node. Data
packets are copied directly from the sendfd to the
receiving fd. The calling program must make sure that
data is removed from the receiving fd in a timely manner.

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>