This is preliminary work, that could be leveraged to generate a summary
file during a “first, fast run” which could be used in a “second, slow
run” to only work on mirrors that weren't already spotted as obviously
not in a good shape.

This makes it possible for the caller to figure out whether things went
fine.

Ignores offenses introduced by eac9d70c,
which was meant to fix offenses that sajolida saw on his side. I suppose
his older Rubocop does not honor these 2 settings, while mine does.
Whatever :)

Fixes regression introduced while moving the options parsing to
a dedicated file.

In most cases the value of this variable is a URL prefix,
rather than an IP address.

That was useful at the time we were starting to introduce USB images.
Let's now assume we're checking for both USB and ISO images.

Now that the corresponding code was extracted to functions that have names, we
don't need to repeat essentially what those names already say.

The only such extra mirror I'm aware of currently serves… Tails 3.5.