r261230 broke the cases where the amount of data to be read is not
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