Add option --dir to mariadb-dump

Similar to --tab, this option will create *.sql file with DDL , and a *.txt file, with tab separated data

Unlike --tab, this will support multiple databases. It will create a directory structure in the directory pointed to by --dir=path parameter, where directory names correspond to databases that are being (maybe partially) backed up.

All options compatible with --tab are also supported with --dir, in particular parallel execution with consistent backup. So the difference between --tab and --dir is

• --dir creates directory structure described above
• --all-databases and --databases are supported.