Make it possible to resume an IUK download from within Tails

reported by users, but we do it only if it’s cheap to implement because it might be superseded if we look into replacing our upgrader.

Downloading target files should be more robust on flaky Internet connections.

E.g. when a target file download times out, it should be retried a few times. Ideally, the download should be resumed using the already downloaded partial content. Possible implentation ideas:

• LWP::UserAgent::ExponentialBackoff - not in Debian, not sure it resumes interrupted downloads)
• LWP::UserAgent::Determined - in Debian, not sure it resumes interrupted downloads)
• code samples that use the Range header when needed

But we currently use Curl so:

• we could switch to the curl CLI tool that has a --retry option
• https://curl.haxx.se/libcurl/c/CURLOPT_RESUME_FROM.html
• https://curl.haxx.se/libcurl/c/CURLOPT_RANGE.html

Alternatively, there could be a button for manually retrying or resuming an interrupted download.

Related issues

• Related to #14544
• Related to #15277
• Has duplicate #5825 (closed)

Original created by @u on 15875 (Redmine)