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/glue/arm/syscall.h

@@ -2,12 +2,13 @@
  * ARM-specific system call details.
  *
  * Copyright (C) 2007 Bahadir Balban
- *
  */
 
 #ifndef __ARM_GLUE_SYSCALL_H__
 #define __ARM_GLUE_SYSCALL_H__
 
+#include <l4/types.h>
+
 /* Only specific call is the trap that gives back the kip address
  * from which other system calls can be discovered. */
 #define L4_TRAP_KIP			0xB4
@@ -52,7 +53,8 @@ typedef struct msg_regs {
 
 /* NOTE:
  * These references are valid only when they have been explicitly set
- * by a kernel entry point, e.g. a system call, a data abort handler.
+ * by a kernel entry point, e.g. a system call, a data abort handler
+ * that imitates a page fault ipc etc.
  */
 #define KTCB_REF_ARG0(ktcb)	(&(ktcb)->syscall_regs->r0)
 #define KTCB_REF_MR0(ktcb)	(&(ktcb)->syscall_regs->r3)