bug#8968: arc-mode 7z writing support

[Eli Zaretskii]

> From: Juri Linkov <juri@jurta.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  8968@debbugs.gnu.org
> Date: Fri, 08 Jul 2011 21:39:27 +0300
> 
> > All GNU/Linux systems will have only p7zip, as that is the ported
> > version of 7-zip. 7z is only available for Windows, and comes with two
> > command line binaries - 7za.exe, which is Free Software, and 7z.exe
> > which is not (due to unRAR support).
> 
> Does this mean that we should use the binary "7z" on GNU/Linux systems
> and "7za" on Windows?

I actually think that we should not use 7z of any kind at all, but if
we must, then use 7zr on GNU/Linux and 7za on Windows, and offer 7z as
a second option.