MFC r346400:
Improve input validation for the socket option IPV6_CHECKSUM.

When using the IPPROTO_IPV6 level socket option IPV6_CHECKSUM on a raw
IPv6 socket, ensure that the value is either -1 or a non-negative even
number.

MFC r346401:
Avoid a buffer overwrite in rip6_output() when computing the checksum
as requested by the user via the IPPROTO_IPV6 level socket option
IPV6_CHECKSUM. The check if there are enough bytes in the packet to
store the checksum at the requested offset was wrong by 1.

MFC r346402:
When a checksum has to be computed for a received IPv6 packet because it
is requested by the application using the IPPROTO_IPV6 level socket option
IPV6_CHECKSUM on a raw socket, ensure that the packet contains enough
bytes to contain the checksum at the specified offset.

MFC r346406:
When an IPv6 packet is received for a raw socket which has the
IPPROTO_IPV6 level socket option IPV6_CHECKSUM enabled and the
checksum check fails, drop the message. Without this fix, an
ICMP6 message was sent indicating a parameter problem.

Thanks to bz@ for suggesting a way to simplify this fix.

Reviewed by: bz@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D19966
Differential Revision: https://reviews.freebsd.org/D19967
Differential Revision: https://reviews.freebsd.org/D19968
Differential Revision: https://reviews.freebsd.org/D19969