Implement BSD calculations

- load_avg, recent_cpu, priority calculations
- reduce frac bits to 14
- ignore thread_set_priority when BSD enabled

src/threads/thread.h

@@ -25,6 +25,10 @@ typedef int tid_t;
 #define PRI_DEFAULT 31                  /* Default priority. */
 #define PRI_MAX 63                      /* Highest priority. */
 
+#define NICE_MIN -20   /* Lowest niceness. */
+#define NICE_DEFAULT 0 /* Default niceness. */
+#define NICE_MAX 20    /* Highest niceness. */
+
 /* A kernel thread or user process.
 
    Each thread structure is stored in its own 4 kB page.  The