Syscall arguments are neveger negative 

The arguments are defined as `unsigned long`:

``` c
typedef struct open_arg_s {
  unsigned long ret;
  unsigned long ptr_filename;
  unsigned long flags;
  unsigned long mode;
} open_arg_s;
```

However the code has a lot of tests for negativeness:

``` c
if (arg->ret >= 0) {
  // This is always executed
}
```
