MFC r364117 by glebius:

ipfw: make the "frag" keyword accept additional options "mf",
"df", "rf" and "offset". This allows to match on specific
bits of ip_off field.

For compatibility reasons lack of keyword means "offset".

Reviewed by: ae
Approved by: glebius
Differential Revision: https://reviews.freebsd.org/D26021

(cherry picked from commit 825398f946221045c565363a0349f68d054d6455)