Add db_show_indexes() and db_show_primary_key() to database.inc

Now we have:

• • db_add_index
  • db_index_exists
  • db_drop_index
• • db_add_primary_key
  • db_drop_primary_key

So we have create, delete and, in one case test. No option to read. db_show_indexes() and db_show_primary_key() would nicely fill that hole in functionality.