pf tests: Add option to send fragmented packets

Add option to send fragmented packets and to properly sniff them by
reassembling them by the sniffer itself.

Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42354

(cherry picked from commit d7c9de2d68ca81c557e069c2b431529cf597886c)

pf: Fix packet reassembly

Don't drop fragmented packets when reassembly is disabled, they can be
matched by rules with "fragment" keyword. Ensure that presence of scrub
rules forces old behaviour.

Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42355

(cherry picked from commit ede5d4ff5b39ccbc193c30fb6c093c7c4de9a464)

pf: Update documentation regarding matching, scrubbing and reassembly

Update pf documentation:

• default behaviour of fragment reassembly
• introduction of scrub option for filter rules
• disadvantages of using the old scrub ruleset
• options supported for match rules
• fix missing list block end
• remove duplicate description of match filter rule
• update example to modern syntax

Reviewed by: kp
Fragments obtained from: OpenBSD
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42270

(cherry picked from commit 5ed470bdb9da6442d6030cf0a7a8493f759fbb43)