Add some conditionally-compilable diagnostics to the RNG. I got

briefly worried that it might not be doing what I thought it was
doing, but examining these diagnostics shows that it is after all, and
now I've written them it would be a shame not to keep them for future
use.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9938 cda61777-01e9-0310-a592-d414129be87e