Kernel: Rework Process::Priority into ThreadPriority

Scheduling priority is now set at the thread level instead of at the
process level.

This is a step towards allowing processes to set different priorities
for threads. There's no userspace API for that yet, since only the main
thread's priority is affected by sched_setparam().

Kernel/Process.h

@@ -44,15 +44,6 @@ public:
     static Vector<pid_t> all_pids();
     static Vector<Process*> all_processes();
 
-    enum Priority : u8 {
-        IdlePriority,
-        FirstPriority = IdlePriority,
-        LowPriority,
-        NormalPriority,
-        HighPriority,
-        LastPriority = HighPriority,
-    };
-
     enum RingLevel : u8 {
         Ring0 = 0,
         Ring3 = 3,
@@ -75,9 +66,6 @@ public:
 
     static Process* from_pid(pid_t);
 
-    void set_priority(Priority p) { m_priority = p; }
-    Priority priority() const { return m_priority; }
-
     const String& name() const { return m_name; }
     pid_t pid() const { return m_pid; }
     pid_t sid() const { return m_sid; }
@@ -351,7 +339,6 @@ private:
     Vector<FileDescriptionAndFlags> m_fds;
 
     RingLevel m_ring { Ring0 };
-    Priority m_priority { NormalPriority };
     u8 m_termination_status { 0 };
     u8 m_termination_signal { 0 };
 
@@ -429,7 +416,7 @@ private:
     Process& m_process;
 };
 
-extern const char* to_string(Process::Priority);
+const char* to_string(ThreadPriority);
 
 extern InlineLinkedList<Process>* g_processes;