keyboard() 'exit' function broken

[Rik]

The keyboard function for debugging stops normal execution and drops the
user back to the Octave prompt where they can inspect variables, perform
calculations to verify intermediate results, etc.  To resume execution
the user types 'exit' or 'quit'. 

Unfortunately in the latest tip typing 'exit' doesn't resume execution
but drops the user completely out of Octave and back to the shell.

Demonstration code:

function myfunc
set_a_variable_for_inspection = 1;
keyboard;
end

Then type 'myfunc' at the prompt which will stop at line 3 and present
the 'debug>' prompt.  Typing 'exit' will drop the user to the shell.

--Rik