innobase_create_key_def(), row_table_got_default_clust_index(),
row_get_mysql_key_number_for_index(): Add a const qualifier.

dict_table_get_first_index(), dict_table_get_next_index(): Define
as a const-preserving macro.  Preserve the old function for UNIV_DEBUG,
but add a const qualifier to the parameter and cast away the constness.