bug#17663: 24.4.50; arc-mode.el has unrar-free hard-coded

[Eli Zaretskii]

> From: Nicolas Petton <nicolas@petton.fr>
> Date: Wed, 30 Sep 2015 10:18:14 +0200
> 
> I just installed a patch in master to use unar and lsar.
> 
> My patch removes the use of the "unrar-free" binary completely as 1) it
> is no longer available in Fedora, Ubuntu and Debian and 2) it has been
> reported not to work properly, but I guess that's up for discussion.

Given the troubled history of the Free Software programs available to
access RAR archives, and its various format versions, I'd prefer that
we had more than one solution for RAR.  Having just one hard-coded
program sounds like a near-term maintenance headache.  Other
possibilities include unrar-free, GNA's unrar, and bsdtar from
libarchive.  I think we should make this more future-proof.  Nicolas,
would you like to work on that?

P.S. Full disclosure: I actually dislike RAR very much, due to its
non-free nature, and wish we've expressed our dislike by not
supporting it at all.  But given its widespread use (which I don't
quite comprehend, given the XZ format's performance), I don't expect
that to fly any time soon.