Nacl requires an implementation of randombytes. The version offered by
nacl has a strange behaviour (e.g. indefinite blocking in case
/dev/urandom is unavailable) and is hard to portably link (.o file in
some lib directory). It is thus replaced with an own implementation
(that does proper error handling by the way).

- handler is moved to example/handler
- example keys can be found in example/keys (which is intended as root
  for HBBP_KEYDIR)

hbbp_keygen generates all four keys private, public for sender and
receiver with the file names used in hbbp[cd] in the current working
directory.