Register with pmf(9).  Thus we register with both dopowerhooks(9) and
pmf(9), but they are mutually exclusive (apm(4) vs apmdev(4)).

Express pmf(9) hooks in terms of dopowerhooks(9) hook - the reverse
would be more natural, but pmf hook signature is hidden behind
PMF_FN_ARGS/PMF_FN_CALL, so just go the least intrusive way for now.

(uwe)