groffer: add "atril" to document viewers

[Ben Pfaff]

Atril is the document viewer that comes with the MATE desktop
environment.  It is included in, for example, Debian GNU/Linux and
installed by default when the MATE environment is included.  The
"groffer" program doesn't know about it.  The following patch enables
support.  I'd appreciate it if this patch, or an equivalent one, could
be included in a future release of groff.

It took me a long time to understand the problem because the error
message displayed when "groffer --pdf" cannot find a PDF viewer is
obscure:

    set mode: no suitable display mode found under pdf; at
    /usr/lib/groff/groffer/main_subs.pl line 1077.

Maybe it should display a more appropriate message, such as "cannot find
a viewer for PDF".

diff -up groff-1.22.4/contrib/groffer/groffer.pl\~ 
groff-1.22.4/contrib/groffer/groffer.pl
--- groff-1.22.4/contrib/groffer/groffer.pl~    2018-10-10 14:44:56.000000000 
-0700
+++ groff-1.22.4/contrib/groffer/groffer.pl     2019-08-20 15:46:58.998952715 
-0700
@@ -179,18 +179,18 @@ our %Viewer_tty = ('DVI' => [],
                  'X' => [],
                 );
 
-our %Viewer_X = ('DVI' => ['kdvi', 'xdvi', 'dvilx'],
+our %Viewer_X = ('DVI' => ['kdvi', 'xdvi', 'dvilx', 'atril'],
                'HTML' => ['konqueror', 'epiphany'. 'mozilla-firefox',
                           'firefox', 'mozilla', 'netscape', 'galeon',
                           'opera', 'amaya','arena', 'mosaic'],
                'XHTML' => ['konqueror', 'epiphany'. 'mozilla-firefox',
                           'firefox', 'mozilla', 'netscape', 'galeon',
                           'opera', 'amaya','arena', 'mosaic'],
-               'PDF' => ['okular', 'kpdf', 'acroread', 'evince',
+               'PDF' => ['okular', 'kpdf', 'acroread', 'evince', 'atril',
                          'xpdf -z 150', 'gpdf', 'xpdf', 'zathura'.
                          'epdfview', 'qpdfview', 'apvlv', 'qpdfview',
                          'kghostview --scale 1.45', 'gv', 'ggv'],
-               'PS' => ['okular', 'evince', 'gv',
+               'PS' => ['okular', 'evince', 'atril', 'gv',
                         'gs', 'gs_x11', 'ghostscript', 'ghostview',
                         'kghostview --scale 1.45', 'ggv', 'kpdf'],
                'TTY' => ['xless'],