refactor(url-lib): write curl output directly to file

[dtardon]

This pull request makes curl write directly to a file instead of using redirection via stdout. The redirection was originally added in 5c274467d6a4959b7f451f9a211ffbb38d69b641 to workaround https://bugzilla.redhat.com/show_bug.cgi?id=989133 (curl would create no file if the source was empty), but I don't see that problem with current curl.

A nice side-effect of this change is that curl shows download progress.

Checklist

• [x] I have tested it locally
• [ ] I have reviewed and updated any documentation if relevant
• [ ] I am providing new code and test(s) for it