Tict TSD-90 - enable keepalive

    Added some debugging over socket failures to transmitter.

    Set the KEEPALIVE option on the sockets - transmitter.c and receiver.c.
       This was found to be necessary within TAIPAN, connections will
       fail after an hour.

    Also need to set system tcp_keepalive_time to a more reasonable value,
      which I did in the system configuration only - through it could
      be done in the code using IMP_Startup files to set the value.
        The original value was 7200 (2 hours) and I set it to 1800 (30 min).
          This ensures the keep-alive is sent before the network box which
           is closing connections does the close.

        Note - a good page on KEEPALIVE
            http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/

    All the above as about AAO JIRA ticket TSD-90 which has more information
     on this.