Cygwin: timerfd: fix overrun computation

Cygwin: timerfd: fix overrun computation

- Drop erroneous initial computation of overrun count in settime
  for absolute non-realtime clocks.  It's repeated in thread_func
  and thus counted twice.

- Fix overrun computation for timestamp offsets being a multiple of
  the timer interval.  The timestamp has to be corrected after the
  first offset, otherwise the correction loop counts the intervals
  again.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>