Restore support for "serial" links, which were used by default

Restore support for "serial" links, which were used by default
for connecting router nodes in the old days.  This will only work
with the most recent kernel code from P4.

Note however that with such "serial" links (ng_iface nodes in
reality) our dear friend quagga has some issues and refuses to
open multicast sockets there, resulting in RIP / OSPF not working
in default configurations.  Interesting that running routed -P ripv2
works without a glitch on those interfaces.

Bug found by:
Submitted by:
Reviewed by:
Approved by:
Obtained from: