pf: reduce IPv6 header parsing code duplication

There were two loops in pf_setup_pdesc() and pf_normalize_ip6()
walking over the IPv6 header chain. Merge them into one loop,
adjust some length checks and fix IPv6 jumbo option handling. Also
allow strange but legal IPv6 packets with plen=0 passing through
pf. IPv6 jumbo packets still get dropped.
testing dhill@; ok mcbride@ henning@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, d68283bbf0
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46925