bug#8968: arc-mode 7z writing support

[Juri Linkov <juri@jurta.org>]

>> 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.
However, some packages can process formats of non-Free archives.

I see the following Debian packages:

* p7zip-rar.deb - non-free rar module for p7zip

  p7zip-rar provides a module for p7zip-full to make 7z able to
  extract RAR files.

* p7zip-full.deb provides:

 - 7za - a standalone version of the 7-zip tool that handles 7z archives
   and other free compression formats.

 - 7z - handles 7z and other free compression formats.

* p7zip.deb provides:

 - 7zr - a standalone minimal version that only handles 7z archives.

 - p7zip - a gzip like wrapper around 7zr.

There is some confusion: the same name `p7zip' is used for the package
and for the program, and the same name `7z' is used for the package,
for the program and for the format name.

The program `p7zip' (a gzip like wrapper) is not suitable for using in
arc-mode.el on GNU/Linux.  That's why I omitted it from searching for
available programs.  On GNU/Linux it will find the wrong program.
But I don't know the situation on cygwin.

The package `p7zip-full' uses the Free Software subset of the algorithms
supported by 7z.  It contains Free Software that has program names `7z',
`7za'.  So it seems correct to use `7z' for search a program to handle
zip archives.

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

p7zip-rar.deb is a separate non-free module for rar archives.