Temporarily disable polling when no processes are active, while I investigate the problem described below.
Temporarily disable polling when no processes are active, while I
investigate the problem described below.

I am seeing some strange livelock on recent -current sources with
a slow box under heavy load, which disappears with this change.
This might suggest some kind of problem (either insufficient locking,
or mishandling of priorities) in the poll_idle thread.

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