musl does a F_SETFD in its fdopendir

while there also add SERVER_PROTOCOL

their return value is no longer used, it's only confusing at this point.

instead of having a flag to discern between two different behaviours in S_SENDING, split that state into S_SENDING_FILE and S_SENDING_CGI (this will also make it easier in the future to add other sending states). While there, also get rid of `goodbye' and make start_reply advance the state machine by itself.