it is especially useful when you need to know the ratio of good vs bad
mirrors (happens during release process)

closes #18540

Fixes Rubocop Metrics/AbcSize offense.

On my sid system, check-mirrors.rb fails on one mirror whose web page
has invalid HTML:

    $ ./check-mirrors.rb --channel ${DIST:?} --allow-multiple --debug --fast tails-amd64-${VERSION:?} --url-prefix https://ftp.nluug.nl/os/Linux/distr/tails/tails/
    version:  tails-amd64-4.15
    fetch:    https://tails.boum.org/inc/trace
    trace:    1611584352

    mirror:   https://ftp.nluug.nl/os/Linux/distr/tails/tails/
    fetch:    https://ftp.nluug.nl/os/Linux/distr/tails/tails//project/trace
    trace:    1611584352
    fetch:    https://ftp.nluug.nl/os/Linux/distr/tails/tails//stable/
    Traceback (most recent call last):
    	7: from ./check-mirrors.rb:447:in `<main>'
    	6: from ./check-mirrors.rb:447:in `each'
    	5: from ./check-mirrors.rb:465:in `block in <main>'
    	4: from ./check-mirrors.rb:200:in `check_versions'
    	3: from ./check-mirrors.rb:187:in `scan_for_links'
    	2: from /usr/lib/ruby/vendor_ruby/nokogiri/html.rb:16:in `HTML'
    	1: from /usr/lib/ruby/vendor_ruby/nokogiri/html/document.rb:215:in `parse'
    /usr/lib/ruby/vendor_ruby/nokogiri/html/document.rb:215:in `read_memory': Parser without recover option encountered error or warning: 30:8: ERROR: Opening and ending tag mismatch: font and b (Nokogiri::XML::SyntaxError)
    zsh: exit 1     ./check-mirrors.rb --channel ${DIST:?} --allow-multiple --debug --fast

To workaround this, enable the Nokogiri RECOVER parser option,
whose documentation reads "Recover from errors":
https://nokogiri.org/rdoc/Nokogiri/XML/ParseOptions.html

By default wget has a timeout of 900 seconds for each of DNS lookup, TCP
connection, and idle time. Shortening it will make the --fast runs more
predictable in duration.

I was hesitating to put an even lower timeout to match better what the
expectation of users might be: I would probably try to restart a
download that was idle for 60 seconds.

But I'm also afraid of triggering too many fast negative.

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.