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/errno.h

@@ -144,6 +144,9 @@
 #define	EOWNERDEAD	130	/* Owner died */
 #define	ENOTRECOVERABLE	131	/* State not recoverable */
 
+/* Codezero specific error codes */
+#define EACTIVE		132	/* Task active */
+
 #ifdef __KERNEL__
 
 /* Should never be seen by user programs */