allow more than one EventSet attach to a CPU at a time
allow more than one EventSet attach to a CPU at a time

This is necessary for perf_event_uncore support, as multiple uncores
will want to attach to a CPU.

It looks like this change won't break anything, and the tests pass
on my test machines.

I am a bit concerned about cpu->running_eventset, though no one
seems to use that value...

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