Add naettGetTotalBytesRead

[erkkah]

Replaces PR #18.

This lets you implement a progress bar for downloads safely and efficiently by calling naettGetTotalBytesRead() after checking naettComplete() and finding out it's not done yet.

The first commit makes sure that res->headers only gets written to once by the various backends. This is not actually needed for the two later commits, and still doesn't technically make it safe to read headers before the full request is complete (although in practice it'll be), but it's still good practice and probably a slight performance improvement. Not implemented in linux.c though because it's less convenient to do there.

Fixes #16

[erkkah]

@hrydgard the problem with the tests was that they were triggered on pull_request_target, which ran then in the context of the target, without the new changes.

I fixed the linux implementation, which measured the wrong byte count. On Windows, there was confusion caused by the _T macro, so I replaced that will a runtime call to winFromUTF8.

Now, it seems to work. Could you verify if it builds on MSVC?

[hrydgard]

Builds fine. Just a single, not very unexpected warning, which we can live with I guess: