bug#14110: 24.3.50; Add command to open files outside of Emacs (use xdg-open, open etc)

[Eli Zaretskii]

> From: Jambunathan K <kjambunathan@gmail.com>
> Date: Thu, 04 Apr 2013 23:22:37 +0530
> Cc: 14110@debbugs.gnu.org
> 
> The alist is already maintained by the user's desktop (likely via
> Explore->`Open with' -> `Always use this application for these files').
> Emacs should at no point in time have any knowledge of this association.

Emacs can easily know about the associations, at least on Windows.

> To accommodate other systems which are non-desktopy (DOS?), we can
> enhance the shell-command history var to be a (cons FILE-EXTENSION
> VIEWER).  This will be an exception rather than the norm.

That's meaningless on DOS, which is a single-process system.