fabricd: add support for completely unnumbered operation

fabricd: add support for completely unnumbered operation

With this commit, fabricd can run without any IPv4 addresses configured
except on loopback. There are two changes to achieve this:

a) If a circuit has no IPv4 address configured, fabricd will resort to
advertise the routers loopback IP in the OpenFabric hellos.

b) All the routes from OpenFabric are sent with ZEBRA_FLAG_ONLINK set,
so that zebra will install them into the fib without checking whether
the nexthop is reachable

Signed-off-by: Christian Franke <chris@opensourcerouting.org>