Go back to linked list instead of tree implementation for ip filter

[mkrrr]

It was not possible to configure an allowed host with the current implementation of the IP filter module. The tree based implementation has a "closest match" in mind whereas we want the filters to be evaluated sequentially.
This undoes the work done c5f02f4 (Task #832. Changed ip filter from queues to trees). Furthermore, the ALLOW filter was removed in this commit, added it back.
Still work in progress, would like a review. Example configuration:

        netmask.addr = ~(netmask.addr & 0);
        any_addr.addr = ~(any_addr.addr & 0);
        pico_ipv4_filter_add(ppp, 0, NULL, NULL, &allow_start_ip, &netmask, 0, 0, 0, 0, FILTER_ALLOW); 
        pico_ipv4_filter_add(ppp, 0, &allow_start_ip, &netmask, NULL, NULL, 0, 0, 0, 0, FILTER_ALLOW); 
        netmask.addr = 0;
        pico_ipv4_filter_add(ppp, 0, NULL, NULL, &any_addr, &netmask, 0, 0, 0, 0, FILTER_DROP);

[frederikvs]

Generally the code looks good. A bit surprising that your rather small commit can undo the work done in a much larger commit like c5f02f4, but then again, that commit seems to be a jumble of different changes all thrown together in the same commit...

a couple of small questions

• Did you run the unit & smoke tests? Did they succeed? If they still succeed after this a modification, we need to add some tests there :-)
• You mention that it's still a work progress, what exactly still needs to happen?

Just one small comment inline.

[frederikvs]

Unless my eyes deceive me, this looks like a use-after-free, no?