mariabackup does not complain about unknown options

When mistyping an option on the mariabackup command line mariabackup does not complain about an unknown option but continues to take the backup.

Examples:
1. Type targetdir= instead of target-dir= --> mariabackup creates a directory xtrabackup_backupfiles in the current working directory and puts the backup there.

2. Type incremental-base-dir= instead of incremental-basedir= --> mariabackup will take a full backup instead

Expected behaviour: mariabackup should throw an error on each unknown option and show the mariabackup --help output and terminate with exit code not equal 0.