web--browse: use *www-browser if available
Debian and derivatives have an alternatives-based default browser
configuration that uses the /usr/bin/gnome-www-browser,
/usr/bin/x-www-browser and /usr/bin/www-browser symlinks.
When no browser is selected by the user and the Debian alternatives are
available, try to see if they are one of our recognized selection and
in the affermative case use it. Otherwise, warn the user about them
being unsupported and move on with the previous detection logic.
Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>