Mark constant 'null_tables' with table->const_table=1

This was done to make thing consistent. It gives the additional benefit
that EXPLAIN EXTENDED now treat null_tables like constant's and replaces
columns with NULL, in a similar way that it replaces columns with constants
for constant tables.

- Null tables are tables where all columns are always NULL. The most common
  NULL TABLE is a table used in a LEFT_JOIN that is never true.
- All result changes comes from replacing columns with NULL for null_tables.
- "Impossible where" is now also shows constants for const columns.
- Removed duplicated s->type= JT_CONST
- Reset found_const_table_map when JOIN is created (safety fix)

sql/sql_select.h

@@ -1436,7 +1436,7 @@ public:
     table_count= 0;
     top_join_tab_count= 0;
     const_tables= 0;
-    const_table_map= 0;
+    const_table_map= found_const_table_map= 0;
     aggr_tables= 0;
     eliminated_tables= 0;
     join_list= 0;