This patch changes flowgrind's timer management to achieve a nanosecond
resolution (precision). In particular, it changes

* the use of gettimeofday() to clock_gettime(). POSIX.1-2008 marks
  gettimeofday() as obsolete
* any use of the 'timeval' struct to the 'timespec' struct
* the use of the select() call to pselect() call
* the use of ctime() to strftime(). POSIX.1-2008 marks ctime() as
  obsolete