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_update.cc

@@ -1183,7 +1183,8 @@ multi_update::initialize_tables(JOIN *join)
 
     /* ok to be on stack as this is not referenced outside of this func */
     Field_string offset(table->file->ref_length, 0, "offset",
-			table, &my_charset_bin);
+			&my_charset_bin);
+    offset.init(table);
     if (!(ifield= new Item_field(((Field *) &offset))))
       DBUG_RETURN(1);
     ifield->maybe_null= 0;