Mutex test progress

- Mutex test added. Forked tasks demonstrate produce/consumer using a
  shared mmap'ed page.
- Added l4lib assembler syscall
- Added forgotten SWI to mutex control offset in syscall page.
- Added mutex head initialization
- Contended child successfully sleeps in a waitqueue.

Issues:
- Somehow the child's produced page buffer is altered at about [4020] offset.
  Parent fails to validate buffer therefore.
- Need to add syncing to test so that parent does not unlock and lock again
  before child has a chance to lock buffer and produce.

include/l4/api/mutex.h

@@ -6,6 +6,8 @@
 #if defined (__KERNEL__)
 #define MUTEX_CONTROL_LOCK		L4_MUTEX_LOCK
 #define MUTEX_CONTROL_UNLOCK		L4_MUTEX_UNLOCK
+
+void init_mutex_queue_head(void);
 #endif
 
 #define L4_MUTEX_LOCK		0