Turn on SO_KEEPALIVE on network connections. Since we limit the number
of connections, we cannot afford to allow "disappeared" client to cause
us to leave one of the 14 connections open and hanging in a read() forever.

(SO_KEEPALIVE causes probe packets to be sent after a few hours of IDLE
time where no data has been transferred. Sup should NEVER do this, so the
only time it will have an effect is if it looses the remote machine)