Use rootlesskit instead of dockerd where relevant

[github-actions]

This is an automatic cherry-pick of e22fd9ed979458dfae7c823494f95d288e17ce64

The original PR was #225 with message:

The PID returned from g_spawn_async() is that of rootlesskit, not dockerd, so it makes sense to talk about the rootlesskit process in the log_debug() statements.

Also changed so 0 rather than -1 means no child process is running.

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