A wrong check was silently skipping the initialization of the bfd_session
struct in the adjacency if the router was not configured for IPv6. This
would cause BFD events to be ignored regardless of the configuration.
Also add a function to return the "name" of an adjacency and use it in a
couple of places, including the new log, instead of repeating the same
code in a bunch of places.
Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>