A sampling PMC in a multiplexed group only retires a sample while its
group holds the hardware. If the period needs more events than one
rotation window can retire, the PMC produces almost no samples, and
pmcstat reported nothing about it.

Warn once, after the run, when the requested period could not fit a
rotation window. Compute the estimate before the run so that the
warning reflects what was asked for.

Report descendants whose group could not follow a fork. The work of
those children is missing from the totals, which otherwise look
complete. Print the group-inherit-miss record in the log dump and in
pmc(8) view.

Sponsored by: AMD