[kernel] Add new zero_ok flag to syscall params

The new zero_ok flag is similar to optional for reference parameters,
but only in cases where there is a length parameter. If that parameter
is a reference parameter itself and is null, or it is non-null and
contains a non-zero length, or there is no length parameter, then the
main parameter may not be null.

scripts/definitions/types/function.py

@@ -62,5 +62,7 @@ class Param:
 
     @property
     def optional(self):
-        return "optional" in self.options
+        if "zero_ok" in self.options: return "zero_ok"
+        elif "optional" in self.options: return "optional"
+        else: return "required"