Towards finishing exchange_registers()

- Added mutex_trylock()
- Implemented most of exchange_registers()
- thread_control() now needs a lock for operations that can modify thread context.
- thread_start() does not initialise scheduler flags, now done in thread_create.

TODO:
- Fork/clone'ed threads should retain their context in tcb, not syscall stack.
- exchange_registers() calls in userspace need cleaning up.

include/l4/lib/mutex.h

@@ -28,6 +28,7 @@ static inline void mutex_init(struct mutex *mutex)
 	INIT_LIST_HEAD(&mutex->wq.task_list);
 }
 
+int mutex_trylock(struct mutex *mutex);
 void mutex_lock(struct mutex *mutex);
 void mutex_unlock(struct mutex *mutex);
 

include/l4/lib/spinlock.h

@@ -20,7 +20,7 @@ static inline void spin_lock_init(struct spinlock *s)
  */
 static inline void spin_lock(struct spinlock *s)
 {
-	preempt_disable();
+	preempt_disable();	/* This must disable local preempt */
 #if defined(CONFIG_SMP)
 	__spin_lock(&s->lock);
 #endif