Kernel+LibC: Report correct scheduling priority limits

The priority range was changed several years ago, but the
userland-reported limits were just forgotten :skeleyak:. Move the thread
priority constants into an API header so that userland can use it
properly.

Kernel/API/POSIX/sched.h

@@ -16,6 +16,12 @@ struct sched_param {
     int sched_priority;
 };
 
+#define THREAD_PRIORITY_MIN 1
+#define THREAD_PRIORITY_LOW 10
+#define THREAD_PRIORITY_NORMAL 30
+#define THREAD_PRIORITY_HIGH 50
+#define THREAD_PRIORITY_MAX 99
+
 #ifdef __cplusplus
 }
 #endif

Kernel/Thread.h

@@ -15,6 +15,7 @@
 #include <AK/Time.h>
 #include <AK/Variant.h>
 #include <AK/Vector.h>
+#include <Kernel/API/POSIX/sched.h>
 #include <Kernel/Arch/RegisterState.h>
 #include <Kernel/Debug.h>
 #include <Kernel/Forward.h>
@@ -46,12 +47,6 @@ struct ThreadSpecificData {
     ThreadSpecificData* self;
 };
 
-#define THREAD_PRIORITY_MIN 1
-#define THREAD_PRIORITY_LOW 10
-#define THREAD_PRIORITY_NORMAL 30
-#define THREAD_PRIORITY_HIGH 50
-#define THREAD_PRIORITY_MAX 99
-
 #define THREAD_AFFINITY_DEFAULT 0xffffffff
 
 struct ThreadRegisters {

Userland/Libraries/LibC/sched.cpp

@@ -20,13 +20,13 @@ int sched_yield()
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sched_get_priority_min.html
 int sched_get_priority_min([[maybe_unused]] int policy)
 {
-    return 0; // Idle
+    return THREAD_PRIORITY_MIN;
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sched_get_priority_max.html
 int sched_get_priority_max([[maybe_unused]] int policy)
 {
-    return 3; // High
+    return THREAD_PRIORITY_MAX;
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/sched_setparam.html