Added a config-main General option to delete sys.exitfunc.  The default
is not to do that.  VPython and student environment support.

M PyShell.py
M config-main.def
M run.py
diff --git a/Lib/idlelib/config-main.def b/Lib/idlelib/config-main.def
index 1350d60..206c37f 100644
--- a/Lib/idlelib/config-main.def
+++ b/Lib/idlelib/config-main.def
@@ -43,6 +43,7 @@
 autosave= 0
 print-command-posix=lpr %s
 print-command-win=start /min notepad /p %s
+delete-exitfunc= 0
 
 [EditorWindow]
 width= 80