Table definition cache, part 2

The table opening process now works the following way:
- Create common TABLE_SHARE object
- Read the .frm file and unpack it into the TABLE_SHARE object
- Create a TABLE object based on the information in the TABLE_SHARE
  object and open a handler to the table object

Other noteworthy changes:
- In TABLE_SHARE the most common strings are now LEX_STRING's
- Better error message when table is not found
- Variable table_cache is now renamed 'table_open_cache'
- New variable 'table_definition_cache' that is the number of table defintions that will be cached
- strxnmov() calls are now fixed to avoid overflows
- strxnmov() will now always add one end \0 to result
- engine objects are now created with a TABLE_SHARE object instead of a TABLE object.
- After creating a field object one must call field->init(table) before using it

- For a busy system this change will give you:
 - Less memory usage for table object
 - Faster opening of tables (if it's has been in use or is in table definition cache)
 - Allow you to cache many table definitions objects
 - Faster drop of table

sql/sql_select.h

@@ -440,10 +440,11 @@ class store_key :public Sql_alloc
   {
     if (field_arg->type() == FIELD_TYPE_BLOB)
     {
-      /* Key segments are always packed with a 2 byte length prefix */
-      to_field=new Field_varstring(ptr, length, 2, (uchar*) null, 1, 
-				   Field::NONE, field_arg->field_name,
-				   field_arg->table, field_arg->charset());
+        /* Key segments are always packed with a 2 byte length prefix */
+      to_field= new Field_varstring(ptr, length, 2, (uchar*) null, 1, 
+                                    Field::NONE, field_arg->field_name,
+                                    field_arg->table->s, field_arg->charset());
+      to_field->init(field_arg->table);
     }
     else
       to_field=field_arg->new_key_field(thd->mem_root, field_arg->table,