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.

tasks/mm0/include/exregs.h

@@ -0,0 +1,22 @@
+#ifndef __MM0_EXREGS_H__
+#define __MM0_EXREGS_H__
+
+#include <l4/api/exregs.h>
+
+void exregs_set_stack(struct exregs_data *s, unsigned long sp);
+void exregs_set_mr_return(struct exregs_data *s, unsigned long retreg);
+void exregs_set_pc(struct exregs_data *s, unsigned long pc);
+
+/*
+exregs_set_stack(unsigned long sp)
+exregs_set_pc(unsigned long pc)
+exregs_set_return(unsigned long retreg)
+exregs_set_arg0(unsigned long arg0)
+exregs_set_mr0(unsigned long mr0)
+exregs_set_mr_sender(unsigned long sender)
+exregs_set_mr_return(unsigned long retreg)
+exregs_set_all(unsigned long arg0, unsigned long arg1, unsigned long arg2, unsigned long arg3,
+	       unsigned long sp, unsigned long pc, u32 valid_vector, l4id_t pager);
+*/
+
+#endif /* __MM0_EXREGS_H__ */