[SHOW] EXPLAIN UPDATE/DELETE, code re-structuring

- Handle another specific case where there the JOIN 
  never had a query plan, but had multiple join->cleanup(full=true) calls
- The idea that there can only be MAX_TABLES subuqeries/unions was 
  wrong. Switch QPF_query to using a Dynamic_array.
  = make Dynamic_array template support size growth. its underlying
    DYNAMIC_ARRAY supports it. (this part will need more polishing)

sql/sql_array.h

@@ -106,6 +106,7 @@ public:
 
   Elem& at(size_t idx)
   {
+    DBUG_ASSERT(idx < array.elements);
     return *(((Elem*)array.buffer) + idx);
   }
 
@@ -139,6 +140,23 @@ public:
     array.elements= n;
   }
 
+  bool resize(size_t new_size, Elem default_val)
+  {
+    size_t old_size= elements();
+    if (allocate_dynamic(&array, new_size))
+      return true;
+    
+    if (new_size > old_size)
+    {
+      set_dynamic(&array, (uchar*)&default_val, new_size - 1);
+      /*for (size_t i= old_size; i != new_size; i++)
+      {
+        at(i)= default_val;
+      }*/
+    }
+    return false;
+  }
+
   ~Dynamic_array()
   {
     delete_dynamic(&array);