Additional fixes for MariaDB backup support

[smeeus]

Command build fixes, argument fixes for MariaDB backup Compression works fine with MariaDB backup MariaDB backup does not have encryption support Reflect proper binary in output Remove double use database sql command Include MariaDB backup info in documentation

This has been tested with similar clusters:

• Galera / MariaDB 10.2.19, compression on, using MariaDB backup, full and differential incrementals
• Galera / MariaDB 10.2.7, compression on, monyog enabled, using Percona XtraBackup 2.4, full and differential incrementals

Can be used as a fix for #37

[jpfeuffer]

This is however probably out of scope for us since we use C++ only.