firewall: link iptables extensions dynamically

firewall: link iptables extensions dynamically

Use shared libipt{,4,6}ext.so libraries instead of statically linking
the userspace matches into the fw3 executable.

As a side effect the match initialization is extremely simplified
compared to the weak function pointer juggling performed before.

This also fixes the initialization of the multiport match.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45764 3c298f89-4303-0410-b956-a3cf2f4a3e73