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.
MFC r264216, r265817, r265822:

Add man page for udplite(4).