Split out NaCl from serval-dna

[dismantl]

The latest version of serval-dna is not only several hundred kilobytes larger than the previous version, it also uses up significantly more memory and CPU cycles. Since we don't currently utilize the MDP overlay feature of serval-dna, and only use it's wrappers of NaCl crypto functions for creating and verifying signatures, it would be of considerable benefit to split out the NaCl functionality currently bundled with serval-dna. This would allow us to include a much smaller library on our router platform, and only include libserval (and operationalizing it as a daemon/plugin) as needed.

The only caveat would be that until there is a unified key space in NaCl, we would need to include public SAS signing keys in service advertisements as additional mDNS TXT entries.