slow: Move version handling to subtype functions

[corubba]

Limit the access of the slow_print function to the subtype field, which is the only field the slow protocol defines. This means moving the version stuff for LACP and Marker to their own dedicated functions.

The output is kept mostly the same as before, which is why there is no change to the test data. The only deliberate change is replacing the "version not supported" message with printing the packet data verbatim.

This refactor was suggested in #1363. While I understand the reason, after seeing the actual change/result, I am personally not entirely convinced it is actually an improvement. But I will provide it for your consideration and judgement.