pf: allow scrub rules without fragment reassemble

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).

Extend 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")
Differential Revision: https://reviews.freebsd.org/D37459