MAINT-1481: remove linux-updater; move logic to Linux update_install

Now that the viewer's own background updater logic is responsible for
downloading a new installer, the only functionality we still use in
linux-updater that couldn't be expressed more simply in bash is the UI. But
since most Linux distros capable of running SL at all have zenity, and all
will have xmessage, we can handle even the UI part. Add xmenity wrapper script
so update_install doesn't have to care which is present, and make the bash
script that used to launch linux-updater do the real work.