Various updates to the release checklist:

 - for 'ixion' read 'atreus' throughout
 - the signature-checking commands needed minor modifications to cope
   with more *sums files
 - stated a few things explicitly which were previously implied, in
   case the next inter-release gap is also long enough for me to
   forget them.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9205 cda61777-01e9-0310-a592-d414129be87e