scrub rules have defaulted to handling fragments for a long time, but
since we removed "fragment crop" and "fragment drop-ovl" in 64b3b4d611
this has become less obvious and more expensive ("reassemble" being the
more expensive option, even if it's the one the vast majority of users
should be using).
This is somewhat unexpected, even if a 'scrub on $interface' rulExtent the 'scrub' syntax to allow fragment reassembly to be didsabled,
show 'scrub on $interface fragment reassemble' when retrieved.
Add a PFRULE_REASSEMBLE_FRAG flag so that we can enable and disablewhile retaining the other scrub behaviour (e.g. TTL changes, random-id,
..) using 'scrub fragment no reassemble'.
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