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.

tasks/test0/main.c

@@ -52,6 +52,9 @@ void main(void)
 		ipc_full_test();
 		ipc_extended_test();
 	}
+	if (parent_of_all == getpid()) {
+		user_mutex_test();
+	}
 	exectest();
 
 	while (1)