Finalise library API

[matwey]

We need to finalise the library API somehow. Only ov_-prefixed functions should be exported. ov.h should be the single one required header file for applications.

[desowin]

It would be good to have function that detects and lists connected OpenVizsla boards. While I believe majority of user will have only one OpenVizsla connected, the proper design would be to enumerate them.

When extcap is implemented, then each device will have its own identifier (specified by extcap itself in the output to --extcap-interfaces call) which will be passed by Wireshark when starting capture as one of the parameters (--extcap-interface).

[matwey]

@desowin Sure. This is how pcap usually works. Every pcap driver provides an interface to enumerate all devices of the given kind.