Add getter for download progress?

[hrydgard]

It would be nice to check the progress of larger downloads.

Something like this:

	if (!naettComplete(res_)) {
		// Update progress
		int downloadedBytes = 0, totalBytes= 0;
		naettGetProgress(res_, &downloadedBytes , &totalBytes);
                // ...
	}

If the size is unknown, *totalSize would be set to -1.

I guess cleanest would be if naettComplete also had two output parameters, downloadedBytes and totalBytes.

Actually I guess this can be hacked by manually reading the Content-Length header, and then checking res->body.size.

Though it occurs to me that there's a data race - if I read headers while !completed, it may be that the headers are currently being filled in.

Here's my current hack that 'works' (doing the below if naettComplete() didn't return true) but has at least two races:

	const char *header = naettGetHeader(res_, "Content-Length");
	if (header) {
		strlcpy(buf, sizeof(buf), header);
		sscanf(buf, "%d", &total);
		int size;
		naettGetBody(res_, &size);
		progress_.Update(size, total);
	}