Add support for slave_skip_errors="ddl_exist_errors"

MySQL supports value "ddl_exist_errors" for option slave_skip_errors, which automatically configures the slave to ignore errors caused by DDL statements trying to create existing objects or drop non-existent ones. MariaDB could implement this option to do the same thing.

Note this JIRA was originally a bug report about this option not updating slave_skip_errors when set, because the MariaDB documentation mistakenly said the option was supported.