Patch to Octave shutdown procedures

[John Swensen]

As I have been working on OctaveDE, I have found that the way Octave  
currently shuts down has made me jump through hoops to get the  
shutdown sequence right.  Because the clean_up_and_exit() function  
directly calls exit() and because I am interacting with octave via a  
different thread, I have had serious issues with variables becoming  
invalid in the middle of an operation.  I propose the following patch  
(or something similar) to allow an IDE to clean up Octave on its own.   
The patch simply provides two additional flags:
1) ide_embedded: allows a user to flag and instance of octave as being  
embedded in an IDE.  This flags makes it so clean_up_and_exit only  
sets the do_exit_readline_loop flag
2) do_exit_readline_loop: this flags causes the main_loop which calls  
readline to exit and do nothing else

Then, in my IDE, after I have stopped all the threads that are  
interacting with Octave and am sure that the readline idle event loop  
is also no longer running, I call do_octave_atexit() myself to clean  
things up.  Maybe you would like a different format or manner of  
handling this, but at this point it is somewhat of a necessity for me  
to create a robust IDE using VTE in which the Octave process is simply  
embedded in a terminal window in my application.  I have spent a lot  
of time trying workarounds and the only other viable option it to  
issue a full KILL signal to Octave running in VTE, which seems ugly  
and unnecessary and doesn't work for Windows.  This patch would work  
for both Windows, Linux, and OSX.

John Swensen

ide_embedded_changeset.patch
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