This patch add new ipfw_pmod kernel module, that currently implements tcp-setmss rule action.
It sets TCP MSS value and supports both IPv4 and IPv6.
tcp-setmss rule works like setdscp rule. All TCP packets with SYN flag are matched, and if packet contains MSS TCP option, and its value is greater than specified in tcp-setmss VALUE, it modifies this value and updates TCP checksum if needed.
New O_EXTERNAL_DATA opcode added. It used together with O_EXTERNAL_ACTION opcode, but it does not require creating of some named instances, like O_EXTERNAL_INSTANCE opcode requires.
Later we can add some another opcodes with similar functional, but for different protocols to this ipfw_pmod module.