MFC r316435:

MFC r316435:
  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
  Relnotes:	yes
  Sponsored by:	Yandex LLC
  Differential Revision:	https://reviews.freebsd.org/D10150