bug#19350: #19350 24.4; Incorrect quoting of %-signs for Windows command shell

[Eli Zaretskii <eliz@gnu.org>]

> From: npostavs@users.sourceforge.net
> Date: Sun, 14 Aug 2016 23:13:43 -0400
> Cc: 19350@debbugs.gnu.org
> 
> Hmm, maybe we could fix this by making Emacs refuse to apply environment
> variables with names ending in carets?

I'm very much against disallowing perfectly valid (if rare) use cases
just because someone malicious can take advantage of that.

From my POV, as long as Emacs itself doesn't produce such shell
commands and/or environment variables for any of its features,
avoiding this becomes user's responsibility, just like when working at
the shell prompt.  Of course, if we can find a reliable solution to
the problem that doesn't take away features, that'd be better, but
failing that, I'm okay with leaving this alone, perhaps documenting
somewhere that using % in shell commands when there are environment
variables ending in ^ could produce surprising results.

Thanks.