libc/sched: Map the nice value more correctly

from https://pubs.opengroup.org/onlinepubs/007904875/functions/setpriority.html:
1.The nice value shall in the range [-{NZERO},{NZERO} -1]
2.Lower nice value shall cause more favorable scheduling

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I5ad60d92abc3b69fbaa406da68cec2e40ca3fa6d

include/limits.h

@@ -276,10 +276,10 @@
 #define NL_TEXTMAX _POSIX2_LINE_MAX
 
 /* NZERO
- *   Default process priority. Minimum Acceptable Value: 20
+ *   Default process priority. Minimum Acceptable Value: 128
  */
 
-#define NZERO 20
+#define NZERO 128
 
 /* Required for asynchronous I/O */
 

libs/libc/unistd/lib_getpriority.c

@@ -82,5 +82,9 @@ int getpriority(int which, id_t who)
       return ret;
     }
 
-  return param.sched_priority;
+  /* Since -1 is a legal return value, clear errno to avoid the chaos */
+
+  set_errno(0);
+
+  return NZERO - param.sched_priority;
 }

libs/libc/unistd/lib_setpriority.c

@@ -69,7 +69,7 @@ int setpriority(int which, id_t who, int value)
       return ret;
     }
 
-  param.sched_priority = value;
+  param.sched_priority = NZERO - value;
 
   return sched_setparam(who, &param);
 }