Make hpath:external-display-alist-x customizable

[Jean Louis]

The variable hpath:external-display-alist-x should be made
customizable, same for other operating systems.

(("\\.e?ps$" . "ghostview") ("\\.dvi$" . "xdvi") 
("\\.\\(doc[mx]?\\|od[st]\\|ppsx?\\|ppt[mx]?\\|xls[mx]?\\)$" . "openoffice") 
("\\.pdf$" "xpdf" "acroread") ("\\.ps\\.g?[zZ]$" . "zcat %s | ghostview -") 
("\\.\\(gif\\|tiff?\\|xpm\\|xbm\\|xwd\\|pm\\|pbm\\|jpe?g\\)" . "xv") 
("\\.ra?s$" . "snapshot -l"))

xv is shareware and thus not free software, I don't think it should be
hard coded in GNU Hyperbole, if I am not mistaken GNU Hyperbole is
part of GNU system, and then it should not point to non-free
software. The image viewer I am using is sxiv, very handy and free
software. There are many others.

There are some new readers and software, often libreoffice is used
rather than openoffice, evince and mupdf and other PDF readers are
today more often used than xpdf or acroread which I do not even have
in my free operating system any more.

Customization would be good for this variable.

I use:  Editor:      GNU Emacs 28.0.50 (build 15, x86_64-pc-linux-gnu, X 
toolkit, cairo version 1.14.8, Xaw3d scroll bars)

        Hyperbole:   7.1.2
        Sys Type:    x86_64-pc-linux-gnu
        OS Type:     gnu/linux
        Window Sys:  x
        News Reader: Gnus v5.13