Add thread and process states.

Add a thread exit call.

zion/scheduler/process.h

@@ -7,6 +7,12 @@ class Thread;
 
 class Process {
  public:
+  enum State {
+    UNSPECIFIED,
+    SETUP,
+    RUNNING,
+    FINISHED,
+  };
   // Caller takes ownership of returned process.
   static Process* RootProcess();
   Process(uint64_t elf_ptr);
@@ -17,10 +23,17 @@ class Process {
   void CreateThread(uint64_t elf_ptr);
   Thread* GetThread(uint64_t tid);
 
+  // Checks the state of all child threads and transitions to
+  // finished if all have finished.
+  void CheckState();
+
+  State GetState() { return state_; }
+
  private:
   Process(uint64_t id, uint64_t cr3) : id_(id), cr3_(cr3) {}
   uint64_t id_;
   uint64_t cr3_;
+  State state_;
 
   uint64_t next_thread_id_ = 0;