Implement BSD calculations

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

src/threads/fixed-point.h

@@ -8,7 +8,7 @@ typedef struct
 } fp32_t;
 
 /* Fixed Point Arithmetic bit count constants */
-#define NUM_FRAC_BITS 20
+#define NUM_FRAC_BITS 14
 #define NUM_INT_BITS (31 - NUM_FRAC_BITS)
 #define CONVERSION_CONST (1 << NUM_FRAC_BITS) /* f = 2^q, (2^20) */