[NETFILTER]: Make conntrack timeouts become sysctls.

[NETFILTER]: Make conntrack timeouts become sysctls.

This patch introduces the /proc/sys/net/ipv4/netfilter/ directory for=20
the tuneable parameters of the conntrack engine of netfilter.

The parameters which can currently be fine-tuned (if required at all)=20
are the ip_conntrack_max, and the conntrack timeout parameters in seconds.

The default timeout value for the TCP CLOSE_WAIT state is raised to 3
days, which should cause less problems with long-lived half-closed TCP
sessions.

For the sake of compatibility with 2.4,=20
/proc/sys/net/ipv4/netfilter/ip_conntrack_max is duplicated as=20
/proc/sys/net/ipv4/ip_conntrack_max as well.