Added Klaus Schleisiek-style frames to startup.mu4
Added Klaus Schleisiek-style frames to startup.mu4

Now we can neatly and reliably nest:

 * the loading of files
 * setting radix, source, first, line
 * room for local variables

and it's possible to have code executed "on exit" - which is how files
that are being read get closed.

All of this works either on normal return, or when the stack is unwound
on exception handling (which doesn't occur - yet).

nickhen/muforth,nickhen/muforth