No very good reason, but I've occasionally wanted to frob it to see if it
makes any difference to problems I'm having, and it was easy.
Tested that it does actually cause keepalives on Windows (with tcpdump);
should also work on Unix. Not implemented on Mac (does nothing), but then
neither is TCP_NODELAY.
Quite a big checkin, much of which is adding `keepalive' alongside `nodelay'
in network function calls.
[originally from svn r4309]
we can have runtime switching between MacTCP and OpenTransport, and so
that we can cope if there's no TCP/IP stack available at all (albeit with
very little functionality at present).
[originally from svn r2546]