Keep "--existing" kernel alive on Ctrl-D exit

[yangle]

When exiting from jupyter-console --existing by pressing Ctrl-D, the expected behavior is that the existing kernel is left alive. See #48 for a discussion. (This might be related to #126, too.)

Currently, however, the kernel is shut down upon console exit. This commit fixes this.

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Apparently, setting self.keepkernel = True affects only the Ctrl-D behavior. The exit command still has keep_kernel=False by default.

Before:

	jupyter-console	jupyter-console --existing
exit	Shutting down kernel	Shutting down kernel
exit(keep_kernel=True)	owning kernel, cannot keep it alive	keeping kernel alive
Pressing Ctrl-D	Shutting down kernel	Shutting down kernel

After:

	jupyter-console	jupyter-console --existing
exit	Shutting down kernel	Shutting down kernel
exit(keep_kernel=True)	owning kernel, cannot keep it alive	keeping kernel alive
Pressing Ctrl-D	owning kernel, cannot keep it alive	keeping kernel alive

[takluyver]

Thanks! One note - if you press Ctrl-D when it does own the kernel, I would rather keep the 'shutting down kernel' message. I think that can easily be done by setting keepkernel to not self.own_kernel. That latter attribute is set by the application when it sets up the shell object.

[yangle]

Indeed, setting self.keepkernel = not self.own_kernel changes the Ctrl-D exit to "Shutting down kernel" when not owning the kernel.

The commit has been updated.

After:

	jupyter-console	jupyter-console --existing
exit	Shutting down kernel	Shutting down kernel
exit(keep_kernel=True)	owning kernel, cannot keep it alive	keeping kernel alive
Pressing Ctrl-D	Shutting down kernel	keeping kernel alive

[takluyver]

Thanks :-)