Add ipfw_pmod kernel module.

The module is designed for modification of a packets of any protocols.
For now it implements only TCP MSS modification. It adds the external
action handler for "tcp-setmss" action.

A rule with tcp-setmss action does additional check for protocol and
TCP flags. If SYN flag is present, it parses TCP options and modifies
MSS option if its value is greater than configured value in the rule.
Then it adjustes TCP checksum if needed. After handling the search
continues with the next rule.

Obtained from: Yandex LLC
MFC after: 2 weeks
Relnotes: yes
Sponsored by: Yandex LLC
No objection from: network
Differential Revision: https://reviews.freebsd.org/D10150