pf: fold pf_test_fragment() into pf_test_rule()

[kp]

Summary

Reduces code and fixes a bunch of bugs with fragment handling not being in sync
with the rest of the ruleset.

Much feedback from mpf, bluhm & markus
Thanks to Tony Sarendal for help with testing

ok bluhm; various previous versions ok henning, claudio, mpf, markus

Note that while this changes the order of src addr/src port/dst addr/dst port
skips this doesn't actually affect the kernel/userspace ABI. The kernel always
recalculates skip steps.

Obtained from: OpenBSD, mcbride <[e-mail]>, 04c69899a7
Sponsored by: Rubicon Communications, LLC ("Netgate")