Add verbosity switch to suppress diagnostic output

This changeset adds a -verbose switch to turn on diagnostic output. If it is not set, only the summary will be displayed.

Things to look at:

• This switch has no effect on the log file's output.
• ioutil.Discard is used instead of io.Discard since the latter one is not available in one of Go versions tested in CI.

(See also: #40079 (closed))