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/api/exregs.h

@@ -0,0 +1,22 @@
+/*
+ * Exchange registers system call data.
+ *
+ * Copyright (C) 2008 Bahadir Balban
+ */
+#ifndef __EXREGS_H__
+#define __EXREGS_H__
+
+#include <l4/macros.h>
+#include INC_GLUE(syscall.h)
+#include INC_GLUE(context.h)
+#include <l4/types.h>
+
+/* Structure passed by userspace pagers for exchanging registers */
+struct exregs_data {
+	exregs_context_t context;
+	u32 valid_vect;
+};
+
+
+
+#endif /* __EXREGS_H__ */