Add a configuration option for TCP keepalives (SO_KEEPALIVE), default off.

Add a configuration option for TCP keepalives (SO_KEEPALIVE), default off.
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.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4309 cda61777-01e9-0310-a592-d414129be87e