Improve dd command lines to show progress

To give feedback on how long it will take or if it’s stuck forever.

In Tails this works:

pv Persistent/iso/tails-amd64-3.2.iso | dd of=/dev/null bs=16M

In macOS people are reporting using:

• Ctrl+T: https://superuser.com/questions/794751/is-there-a-way-to-output-the-status-of-dd-on-os-x-during-the-copy-process
• status=progress: https://linuxcommando.blogspot.hu/2008/06/show-progress-during-dd-copy.html
• pv: https://blog.ciarand.me/posts/using-dd-and-pv-on-osx/index.html

As a consequence it should be possible to remove the mention of “the command prompt” which is jargon.

Parent Task: #14548

Original created by @sajolida on 15099 (Redmine)