MEDIUM: listener: use protocol->accept_conn() to accept a connection

MEDIUM: listener: use protocol->accept_conn() to accept a connection

Now listener_accept() doesn't have to deal with the incoming FD anymore
(except for a little bit of side band stuff). It directly retrieves a
valid connection from the protocol layer, or receives a well-defined
error code that helps it decide how to proceed. This removes a lot of
hardly maintainable low-level code and opens the function to receive
new protocol stacks.