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.

[originally from svn r4309]

unix/uxproxy.c

@@ -221,8 +221,8 @@ static int localproxy_select_result(int fd, int event)
 
 Socket platform_new_connection(SockAddr addr, char *hostname,
 			       int port, int privport,
-			       int oobinline, int nodelay, Plug plug,
-			       const Config *cfg)
+			       int oobinline, int nodelay, int keepalive,
+			       Plug plug, const Config *cfg)
 {
     char *cmd;