change the global variable 'global_flow_cnt's write operation to atomic

Add more `make` options for check, debug, and load.

add options for debug build

replace dynamic malloc of `pkt_node` with a static `pkt_node` pool

rm the redundant `tcp_cc` enum

footer: replace `stack_type` and `tcp_cc` in flow_list with their names

apply commit 81f08f303805bcfd from the stock FreeBSD/siftr.c

cleanup redundant header files

add a new value of field `ipver` per flow in the flow_list

add IPv6 support

beginning version 2.5 for both IPv4 and IPv6 support

remove the `M_ZERO` flag on malloc of "struct pkt_node"

apply the siftr_flowid_filter earlier

poke the consumer only when the ring reaches a threshold (25% full)

add a web wiki link about performance test result