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.

This patch uses the NaCL library to en-/decrypt and authenticate the
payload suitable private and public keys are found. It
- adds crypto.h, containing all crypto logic (including the detection
  if the packet is to be en-/decrypted)
- adds nacl dependency to Makefile (including a switch between
  internal and external nacl)

It also
- changes Makefile to build with -Werror -Wextra
- builds warning-free

referring to the implemented home brew broadcast protocol

builds now when placed in backfire/package
untested if used with feeds