. rename proto to pppox_protos, even being static this is too generic a name
. use rc as the name for result variables, just for consistency with other
  net sources

This is in preparation for having a proper net family module level modules
infrastructure, with the top level (af_pppox) doing the module refcounting
before calling any functions registered by the lower level protocol modules
(in this case just PPPOE for now).