Solve http buffering issues and hangs once and for all (hopefully!) by
simply not trying to return exactly what the caller asked for - just
return whatever we got and let the caller be the judge of whether it
was enough.  If an error occurs or the connection times out after we
already received some data, return a short read, under the assumption
that the next call will fail or time out before we read anything.

As it turns out, none of the code that calls fetch_read() assumes an
all-or-nothing result anyway, except for a couple of lines where we
read the CR LF at the end of a hunk in HTTP hunked encoding, so the
changes outside of fetch_read() and http_readfn() are minimal.

While there, replace select(2) with poll(2).

MFC after:	3 days