Merge branch 'master' of https://github.com/bvanassche/fio into master

Merge branch 'master' of https://github.com/bvanassche/fio into master

* 'master' of https://github.com/bvanassche/fio:
  helper_thread: Increase timer accuracy
  helper_thread: Rework the interval timer code
  Change the return value of two functions from 'void' into 'int'
  helper_thread: Introduce the wait_for_action() function
  helper_thread: Introduce a function for blocking Unix signals
  gettime: Introduce rel_time_since()
  gettime: Simplify get_cycles_per_msec()
  gettime: Introduce fio_get_mono_time()
  configure: Remove the CLOCK_MONOTONIC_PRECISE probe