MINOR: freq_ctr: add a generic function to report the total value

MINOR: freq_ctr: add a generic function to report the total value

Most of the functions designed to read a counter over a period go through
the same complex loop and only differ in the way they use the returned
values, so it was worth implementing all this into freq_ctr_total() which
returns the total number of events over a period so that the caller can
finish its operation using a divide or a remaining time calculation. As
a special case, read_freq_ctr_period() doesn't take pending events but
requires to enable an anti-flapping correction at very low frequencies.
Thus the function implements it when pend<0.

Thanks to this function it will be possible to reimplement the other ones
as inline and merge the per-second ones with the arbitrary period ones
without always adding the cost of a 64 bit divide.