On startup (DESKTOP.SYSTEM), the current ProDOS QUIT handler
(selector/dispatcher) living in Main LC Bank 2 $D100-$D3DD is written
out to a "Quit.tmp" file, along with preamble code to restore it
and invoke QUIT.
On quit (DESKTOP2), the "Quit.tmp" file is loaded at $1000, /RAM is
reattached, then the code is invoked.
This presumes prefix is set on launch and remains unchanged. If
the Quit.tmp file cannot be found, File > Quit shows an error.
Also copy Quit.tmp to RAM Card
