Use xt_id match to track own rules

Use xt_id match to track own rules

Instead of relying on the delegate_* chains to isolate own toplevel
rules from user supplied ones, use the xt_id match to attach a magic
value to fw3 rules which allows selective cleanup regardless of the
container chain.

Also add an experimental "fw3 gc" call to garbage collect empty chains.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>