RFC: pmbootstrap: add audio bell on prompts

Fixes #1911 (closed)

This uses \a to notify the user.

Using xfce4-terminal, there seems to be no detection about whether the terminal has focus. So it rings every time which is really annoying.
According to #1911 (comment 474103244) other emulators behave different.

Maybe @HenriDellal idea to detect the focus would be better.

Apparently there is a "visual bell", but I am not sure what it looks like, or how to trigger it.