fix libCurl download

- Use same URL/zip as windows installer.
- get_dlibcurl32 fails, because the downloaded libcurl.dll
  depends on additional dlls.
- Use std.net.curl to download the zip, i.e. create_dmd_release
  depends on a working libcurl.dll, but uses the downloaded one
  to build a release.
Merge pull request #35 from MartinNowak/fetchCurl

[2.065] fix libCurl download