bug#8968: arc-mode 7z writing support

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@jurta.org>
> Cc: monnier@iro.umontreal.ca,  8968@debbugs.gnu.org
> Date: Fri, 08 Jul 2011 23:20:43 +0300
> 
> >> 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.
> 
> I'd like to find the definitive answer.  On fencepost.gnu.org
> 7z is not installed, but for what reason?

I don't have a definitive answer.  All I know is that arc-mode could
not handle certain archives that I could handle with a 7z.exe
downloaded for comparison.  I then researched this issue and found out
that those archives used a newer format that the Free Software
variants of 7z did not support.  In particular, the variant installed
on fencepost didn't support it.  And the reason was that the newer
formats used code that wasn't Free Software and whose sources were not
available even under non-Free licenses.

> 2. Browse and update zip and other free archives.
> 
> Only `7z' can be used for this functionality, not `7zr'.

Then perhaps we shouldn't offer 7z at all for this purpose.  If we
must, it should be a distant second or third alternative to `zip'.