Task Validation

- Fixed validation bug whereby recurring child tasks inherited the
  parent status ("recurring") instead of the expected "pending".
- Modified 'add' command to code duplicated in Task::validate.
- Cleaned up associated bug.period.t unit test.

src/Task.cpp

@@ -996,8 +996,10 @@ void Task::validate ()
   if (has ("due") &&
       has ("recur"))
   {
-    setStatus (Task::recurring);
-    set ("mask", "");
+    if (has ("parent"))
+      setStatus (Task::pending);
+    else
+      setStatus (Task::recurring);
   }
 
   // Tasks with a wait: date get a special status.