pf: align option handling for IPv4 and IPv6

Block IPv6 packets in pf(4) that have hop-by-hop options header or
destination options header. Such packets can be passed by adding
"allow-opts" to the rule. So IPv6 options are handled like their
counterpart in IPv4 now.
tested by benno@; OK henning@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, c50c83ac27
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D50662