bug#8968: arc-mode 7z writing support

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@jurta.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  8968@debbugs.gnu.org
> Date: Fri, 08 Jul 2011 21:38:17 +0300
> 
> >> So by supporting only 7z, let alone making that the default, Emacs is
> >> in fact preferring a non-free software program to a free one.  We have
> >
> > Oh, I didn't realize 7z was non-Free.  We need to fix that, then.
> 
> AFAIU, 7z as a program is Free Software on GNU/Linux systems.

Not AFAIK.  At least fencepost.gnu.org doesn't have 7z, but does have
p7zip, which is a shell script.  That shell script invokes another
shell script, called 7zr, which in turn invokes a program
/usr/lib/p7zip/7zr.

>  - 7zr - a standalone minimal version that only handles 7z archives.
> 
>  - p7zip - a gzip like wrapper around 7zr.

That's what fencepost has.

> The program `p7zip' (a gzip like wrapper) is not suitable for using in
> arc-mode.el on GNU/Linux.

Why not?

Is 7zr suitable?  If so, we should use it in preference to 7z.

> The difference between programs `7z' and `7za' is not essential:
> one of them is just a standalone version of another.

That's not what Jason said.