That would previously result in the worker getting killed with SIGPIPE.
Which works, but we can also recover from that error without restarting
the process.
I initially planned to only implement the bare minimum to support
FastCGI under nginx, but ended up implementing the full protocol
instead. This is more code than I had expected and the code is also less
trivial than I had hoped. Will need to do more testing, pretty sure
there's bugs left.
Also TODO: test under alternative process managers + document
FU_LISTEN_PROTO.
I've also removed the max_request_body setting, this is something that
really ought to be configured in the web server instead.
