fixes and additions

[lexelby]

This is based on @horkko's #2 and adds some more things that I needed:

• use dates instead of day names to avoid confusion with UTC
• fix up some argument errors
• echo all commands before running them (both to log and stdout)
• extra_backup_args so that I can add --galera-info
• fix #7
• catch mbstream errors that still produce zero exit status

[vmerz]

Works perfect, tested with mariadb1:10.3.29-0 & debian buster. Thank you for your improvements. If i had something to say, i would vote for the pull request.