23090 - Possible thread leak in Shell

In the past when Shell was called from a long running thread the guard
thread spawned by systemu would be blocked in a sleep forever.

This commit attempts to kill the guard thread after systemu has
finished.
Merge pull request #127 from ploubser/bug/master/23090

23090 - Possible thread leak in Shell
23090 - Possible thread leak in Shell

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