[Cryptography] Secure erasure in C.

[Salz, Rich]

> On Windows platforms, Microsoft has a library that offers some form of secure erasure but I haven't used it directly. Instead I rely on the crypto implementations using it.

> Documenting support for similar features in OSX and *nix would be a very useful community service. It might well be necessary to write them first :-).

OpenSSL uses assembler when possible and defaults to memset via volatile pointer when not.  Just use that zeroizing routine for maximum portability?

(Side note:  haven't we had enough of C and language discussions here?)