add support for variable-length checksum in DCCP, as per section 9 of RFC 4340.
remove duplicated code in dccp6_cksum (which just repeated the code of in_cksum
fix a bug in dccp.h -- the fields of CCVAL and CSCOV were swapped (see section 5.1 of RFC 4340)
fix a typo in the display of incorrect checksums
old output `cksum xDEAD (incorrect (-> xBEEF)',
new output `cksum xDEAD (incorrect -> xBEEF)'
variables in the middle of the block., and #if out dccp_hdr_data()
(which is unused, and defined in such a fashion that it doesn't work on
MSVC++, with the structure name and the function name the same).