Extend EID scheme support and validations

Previously uD3TN implicitly assumed string EID in many places such as the agent manager. This adds proper handling for ipn-scheme EIDs, using numerical service numbers as agent IDs in case uD3TN is started with an ipn-scheme EID. If the ipn scheme is used, uD3TN always has to be configured with the service number 0. The config and management agent IDs can be configured in config.h and are 9000 and 9001, respectively, to keep the lower range of service numbers free. It is intentional that agents can register with a service number of zero - uD3TN does not provide specific functionality under this service number.

Additionally, validation of dtn and ipn EIDs is performed according to draft-ietf-dtn-bpbis-31. New helper functions for validation and decoding of EIDs are provided via eid.h.

Closes: #20 (closed), #57 (closed)