bug#8968: arc-mode 7z writing support

[Jason Rumney <jasonr@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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).

Seems I jumped the gun here. p7zip may be the name of the port, but it
also provides 7z and 7za binaries.

>> 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.

I suggest to list in the order 7za, 7zr, 7z.  7za is also available as
part of the p7zip-full package on Debian derived distributions (at
least), so 7zr is only required for systems that only have the minimal
p7zip package installed.