portus and libccp api version numbers

[fcangialosi]

we should tag versions for the interface between portus and libccp. when portus starts up, it should send it's version to the datapath, and only be allowed to proceed if the version matches. the main annoyance is that we need to remember to bump this version each time we change the api, and i'm not sure if there's a more elegant way than just keeping a constant inside both that we try to remember to update manually.