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`? Unless there has been some policy change that I'm unaware of, that module should only contain Windows API functions, i.e. those that you would find in the `Windows.h` C header.

`setenv` and `unsetenv` are by no means standard Windows functions. They're not even in Microsoft's C runtime library, nor any other C implementation for Windows that I know of. These prototypes should probably just be deprecated on Windows altogether (or simply removed).