Responding to Lex's code-review comments.

Capture actual error output from mkdir and mv; display it to user.
Introduce mysudo function used by sudo_mv function for graphical sudo command.
Since update_install actually only displays a single status message, just use
zenity --info instead of a zenity progress box: need not update its message.
Borrow semantics for clear_message and status functions from xmenity script.
Introduce errorbox function so we only have to make zenity/xmessage test once.
Move cleanup, onexit to top so we can use for clear_message.

Test for existence of target name using -e rather than -f. (-d would work too,
but in this case we must respond to any name collision, whether file or
directory.)
Instead of terminating on failure, make sudo_mv return rc of the [sudo] mv
command to its caller. If the attempt to move new install to actual viewer
directory fails, restore previous viewer before failing.
When redirecting the script's stderr to updater.log, first save existing
stderr to another file descriptor, and restore it when we launch viewer.
Otherwise updater.log ends up collecting the viewer's duplicate stderr log
output!
The construct 'exec ... | program' doesn't work. In fact it causes any other
redirections on that command to fail too. Remove it -- real fix pending.

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.

- Fixed an issue where a slash was prepended to the beginning of the filename (which happened to be the full path)