row_merge_create_temporary_table(): Add the parameter index_defs.

DB_PRIMARY_KEY_IS_NULL: New error code, mapped to ER_PRIMARY_CANT_HAVE_NULL.

row_merge_read_clustered_index(): Replace the parameter "table" with the
two parameters "old_table" and "new_table".  Detect NULL values of columns
that are supposed to be NOT NULL.

row_merge_col_prtype(): New auxiliary function, used by
row_merge_create_temporary_table() to flag NOT NULL any columns
contained in an added PRIMARY KEY.