pf: allow scrub rules without fragment reassemble

[kp]

Summary

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).

Extent the 'scrub' syntax to allow fragment reassembly to be disabled,
while retaining the other scrub behaviour (e.g. TTL changes, random-id,
..) using 'scrub fragment no reassemble'.

Sponsored by: Rubicon Communications, LLC ("Netgate")

[mjg]

that seems like a POLA violation which will go unnoticed

how about instead requiring a policy? people will complain about configs which no longer autoload, but that's better than behavior changing without easily visible side effects.

that's assuming the change is worth it to begin with

[kp]

In D37459#851798, @mjg wrote:

that seems like a POLA violation which will go unnoticed

how about instead requiring a policy? people will complain about configs which no longer autoload, but that's better than behavior changing without easily visible side effects.

that's assuming the change is worth it to begin with

The way it's currently documented is a bit ambiguous, but it could certainly be read as documenting the previous behaviour.
Perhaps the way to go is to support a 'scrub fragment noreassemble' to turn reassembly off, leaving the previous behaviour intact, but allowing users to opt out of reassembly while preserving the other scrub actions.

I'll update this patch to do that instead.