Clean up the environment variables created by the parent process for …

[David-wanghw]

…the next call.

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[NorseGaud]

This seems to work, but I have no idea what the scenario is to test. @David-wanghw , any ideas?

Also, shouldn't d.clearEnv() come at the top of the child function?

[sevlyar]

The change may break backward compatibility: function WasReborn() won't work properly after Reborn() call.