Suggested changes to PurpleCipherOps and need for code review

[Ethan Blanton]

Tomasz Wasilczyk spake unto us the following wisdom:
> I did all changes I posted here, but I would like to do one more.
> 
> Currently, purple_cipher_context_reset is hardly usable, because it
> resets both state (IV, digest context) and configuration (key, ops).
> When using simple ciphers, like md5, it's not a problem, but for hmac
> it is - we need to set key and hash type every time, when we want to
> digest a value. I suggest doing a void, but significant change in API:

[snip reset that only resets some values]

I think this should probably be a separate function.  They serve
different purposes.

Ethan