Enable keepalive on TCP connections

[luke-jr]

This helps a lot.

[wrog]

Is there something in particular that this helps with?

Seems like it should be an option and probably a connection option, not just a server option. The problem is keep-alive does cause connections to get dropped when they otherwise wouldn't, i.e., if there's a temporary network outage during which nothing's actually getting transmitted on a connection a keep-alive during that outage would kill the connection, whereas without the keep-aliave if the network came back before any actual packets get sent, it'll be as if there'd never been an outage. And it seems like this would particularly be a problem for places like LambdaMOO where people typically stay idle for very long periods of time.

Should probably also have a way to mess with tcp_keepalive_time and tcp_keepalive_intvl (default for tcp_keepalive_time on my system is 2 hours, which I suspect would make it useless for any application that doesn't customize)