The problem was the mysql_derived_prepare() did not correctly set
'distinct' when creating a temporary derivated table.

Fixed by separating checking for distinct for queries with and without
UNION.

Other things:
- Fixed bug in generate_derived_keys_for_table() where we set the wrong
  bit for join_tab->keys
- Cleaned up JOIN::drop_unused_derived_keys()
- Changed TABLE::use_index() to keep unique keys and update
  share->key_parts

Author: Sergei Petrunia <sergey@mariadb.com>, monty@mariadb.org