rcnetwork: use service type oneshot, not forking

Sometimes, network fails to start / stops itself, because
the network script does execute ifup any more but triggers
"systemctl start network@iface" for each interface, which
have "Requisite=network.service" set and fail, because the
network did not reached started state yet -- it starts.
This does not happen with oneshot service type (bnc#853908).