known in advance, or where the caller doesn't care and just keeps
reading until it hits EOF.
In fetch_read(): the socket is non-blocking, so read() will return 0
on EOF, and -1 (errno == EAGAIN) when the connection is still open but
there is no data waiting. In the first case, we should immediately
return 0. The EINTR case was also broken, although not in a way that
matters.
In fetch_writev(): use timersub() and timercmp() as in fetch_read().
In http_fillbuf(): set errno to a sensible value when an invalid chunk
header is encountered.
In http_readfn(): as in fetch_read(), a zero return from down the
stack indicates EOF, not an error. Furthermore, when io->error is
EINTR, clear it (but no errno) before returning so the caller can
retry after dealing with the interrupt.
MFC after: 3 days
9c1ca3a1dd