Ensure BIRD gets all kernel routes in its table.

If BIRD doesn't have the default route in its routing table it can get
astonishingly confused about what routes are reachable. This change
ensures that, rather than filtering the default route out when its
imported from the kernel, we filter it out when exporting the route via
BGP.

This ensures that BIRD can make the correct decisions about routing,
without confusing every other node.
Ensure BIRD6 can make correct routing decisions.

This is the same fix as 08c0741 for BIRD6. Weirdly, BIRD6 was missing
default route filters. This didn't seem to cause a problem in the systems
we've used, but it looks odd so I added a safety check for it anyway.
Merge pull request #65 from Metaswitch/birdconfig

Update BIRD configuration.