* do not require just only ip6 proto for flow-id opcode in ipfw(8).
ipv6-icmp, tcp, udp should be fine too.
* fix off-by-one bug leading to out-of-bounds read.
* apply IPV6_FLOWLABEL_MASK before comparison in flow6id_match(),
so flow-id opcode will match a specified flow label. No need to
take protocol version and traffic class into account.
* add the test to verify that opcode is working correctly.
Reviewed by: pouria
Obtained from: Yandex LLC
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56869
Boris Lytochkin
3d39eadcde