initial subscribe/unsubscribe API implementation

- add notifier structure to bfd sessions (notifiers are
  not called on state transitions, yet)
- initial (untested) implementation of bfdSubscribe and
  bfdUnsubscribe
- fix memory ownership leak between core and bfd/bfdd
  processes
- renamed bfdRegisterSession to bfdCreateSession and added
  bfdDeleteSession
Merge branch 'master' of github.com:silpertan/FreeBFD