Serval improvements

[dismantl]

overhauled serval-dna plugin crypto code. the only visible change for the user is that the signing and verifying commands give more output, including keys.

to test, make sure app advertising works, and that olsrd-mdp works.

includes https://github.com/opentechinstitute/luci-commotion-apps/pull/68

[jheretic]

@dismantl, with this pull request added, olsr-mdp works as expected. However, there is a problem with application advertisements. Only local advertisements show up, and when running commotion-service-manager manually it gives the following relevant output:

[LOG_INFO] (commotion-service-manager.c:522) Browser: NEW: service 'fb2064837f8946ba4fdaf0f60f4fbefae632c72f' of type '_commotion._tcp' in domain 'mesh.local'
[LOG_INFO] (commotion-service-manager.c:522) Browser: NEW: service 'fb2064837f8946ba4fdaf0f60f4fbefae632c72f' of type '_commotion._tcp' in domain 'mesh.local'
[LOG_WARNING] (commotion-service-manager.c:439: errno: None) (Resolver) Invalid signature: fb2064837f8946ba4fdaf0f60f4fbefae632c72f -> A
[LOG_INFO] (commotion-service-manager.c:129) Removing service announcement: fb2064837f8946ba4fdaf0f60f4fbefae632c72f
[LOG_INFO] (uci-utils.c:332) Application is local

Apparently, even local advertisements are not actually being validated. Remote advertisements give the same output (with the exception of the 'Application is local' part). Running commotiond from the commandline in parallel gives no particular output, but it was not compiled with debugging.