Introduce a new utility function smemclr(), which memsets things to

zero but does it in such a way that over-clever compilers hopefully
won't helpfully optimise the call away if you do it just before
freeing something or letting it go out of scope. Use this for
(hopefully) every memset whose job is to destroy sensitive data that
might otherwise be left lying around in the process's memory.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9586 cda61777-01e9-0310-a592-d414129be87e