Add preinst and postrm scripts to cope with xdg-utils
Now that steamrt-container-host-compat provides an xdg-open symlink, it
conflicts with xdg-utils because /usr/bin/xdg-open
already exists in
the filesystem.
For this reason we divert the real xdg-open of xdg-utils.
This needs to be backported to Soldier too, and to Scout after !98 (merged)