bug#46609: Fix shell password prompt in minibuffer (bug 43302)

[Eli Zaretskii]

> Date: Thu, 18 Feb 2021 01:13:58 +0000
> From:  Ryan Prior via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> I am not a Unicode expert and don't know if there might be undesirable
> side-effects from using :space: instead of :blank:. However, in my
> manual testing these change give me the exact behavior I'm after.

I'm a bit worried by the [[:space:]]* part: it would match any number
of newlines and ^L characters, right?  Is that what we want here?

Perhaps it would be safer to just add a literal newline, and only
once?  Especially for the emacs-27 branch?