This patch adds a new keyword EXTENDED to SHOW TABLES syntax
enabling users to list hidden tables that are created by ALTER
TABLE command. In rare situations ALTER TABLE command might fail
by leaving metadata of '#sql' temporary tables in DD table.

The patch does following changes,

- Change syntax of SHOW TABLES to add EXTENDED command.
  The new syntax would look like,

  SHOW [EXTENDED] [FULL] TABLES ....

- Introduces new native function IS_VISIBLE_DD_OBJECT(). Which
  skips hidden tables, columns, indexes and index elements by
  default.  And show them if EXTENDED keyword is provided in SHOW
  command.

- Change CAN_ACCESS_TABLE()/CAN_ACCESS_COLUMN() native function
  to avoid handling hidden DD object checks. Move the
  responsibility to IS_VISIBLE_DD_OBJECT() native function.

- Enable DROP TABLE command to drop temporary tables left by
  ALTER TABLE.

- Add new test case to verify SHOW EXTENDED and the possibility
  of DROP TABLE to clean left-over temporary tables by ALTER TABLE.