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/arch/arm/exception.h

@@ -75,6 +75,17 @@ static inline void irq_local_disable()
 /* This is filled on entry to irq handler, only if a process was interrupted.*/
 extern unsigned int preempted_psr;
 
+#include <l4/generic/tcb.h>
+static inline int task_in_kernel(struct tcb *t)
+{
+	return ((t->context.spsr & ARM_MODE_MASK) == ARM_MODE_SVC) ? 1 : 0;
+}
+
+static inline int task_in_user(struct tcb *t)
+{
+	return !task_in_kernel(t);
+}
+
 static inline int in_kernel()
 {
 	return (((preempted_psr & ARM_MODE_MASK) == ARM_MODE_SVC)) ? 1 : 0;