shell: Add a special 'zz' global for doing demos and debugging
shell: Add a special 'zz' global for doing demos and debugging

There's now a global 'zz' variable in the shell module, for use
from the console. When a value is assigned to it, it dumps promises
properly, completion, failure and streaming.

So you can do things like this in the javascript console:

    zz = cockpit.spawn(["ping", "127.0.0.1"])

And see the output, failure etc..

Closes #3524
Reviewed-by: Marius Vollmer <bb11c6e942c5cc765df5f04da146b3198127a7a3@redhat.com>

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