NF: Add iptables IDLETIMER target

Each matching packet resets the timer associated with input and/or output
interfaces. Timer expiry causes a kobject uevent. Idle timer can be read via
sysfs.

Signed-off-by: Hiroshi DOYU <hiroshi.doyu@nokia.com>
Signed-off-by: Juha Yrjölä <juha.yrjola@nokia.com>
Updated for 2.6.21-rc1 by Arnaud Patard.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>