Improve thread reap handling

It's a bit of a mess currently, streamline and clean it up.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index c1c7146..7a4b1d1 100644
--- a/fio.h
+++ b/fio.h
@@ -577,6 +577,7 @@
 extern void rate_throttle(struct thread_data *, unsigned long, unsigned int, int);
 extern void fill_start_time(struct timeval *);
 extern void fio_gettime(struct timeval *, void *);
+extern void set_genesis_time(void);
 
 /*
  * Init functions