Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r264212,r264213,r264248,r265776,r265811,r265909:

- Add support for UDP-Lite protocol (RFC 3828) to IPv4 and IPv6 stacks.
Tested with vlc and a test suite [1].
[1] http://www.erg.abdn.ac.uk/~gerrit/udp-lite/files/udplite_linux.tar.gz

Reviewed by: jhb, glebius, adrian

- Fix a logic bug which prevented the sending of UDP packet with 0 checksum.

- Disable TX checksum offload for UDP-Lite completely. It wasn't used for
partial checksum coverage, but even for full checksum coverage it doesn't
work.

Add support for UDP-Lite protocol (RFC 3828) to IPv4 and IPv6 stacks.
Tested with vlc and a test suite [1].

[1] http://www.erg.abdn.ac.uk/~gerrit/udp-lite/files/udplite_linux.tar.gz

Reviewed by: jhb, glebius, adrian