Improve download length detection

Make download length detection, and download progress UI, more robust:

- Use enclosure length if provided
- Use Content-Length if provided for compressed HTTP content too
- Revert the UI to pulsing if expected length is exceeded

Content-Length may be less than the actual payload in presence of
Content-Encoding, but probably only slightly so, and only for .exe
installers, so using it is preferable to not having any size information
at all.