WIP: Deprecate std.c.* and move all remaining declarations to core.stdc.*

I'm still working on this, thought I'd get the first cut of the low-hanging fruit up on the auto-tester before I move forward with the more involved modules.  Ultimately, it will need to be merged simultaneously as [druntime pull #893](https://github.com/D-Programming-Language/druntime/pull/893).

I used the same process to deprecate `std.c.string` recently.  If anybody sees any problems with the pull so far, please speak up.
>Why have you placed the setenv and unsetenv prototypes in core.sys.windows.windows?

I was unsure where those actually went or if they were even defined on Windows.  I've been putting off looking it up, thanks for doing it for me. :) If they weren't available from the native Windows libraries, I thought they might be used by `MinGW`, so I didn't want to break anything by getting rid of them.

Rest assured, this pull is a work in progress and I'm not done figuring out what to do with them.