update Backup script for mysql

[sseide]

• Url parsing is a bit unreliable - especially as one might (need) to set some extra config parameters via connection url like mysql://server:3306/mmonit?serverTimezone=UTC&...
  (https://mmonit.com/wiki/MMonit/Setup#database -> database url format)
  For cli exists a little tool from the author of cURL that is available as OS packages starting with Alpine 3.19 / Debian 13 Trixie:
  https://pkgs.alpinelinux.org/packages?name=trurl&branch=v3.19&repo=&arch=&maintainer=
  https://daniel.haxx.se/blog/2023/04/03/introducing-trurl/
  unfortunately it is not part of Alpine 3.18

• the script cd'sinto the /config folder. The env var backup_filename holds the file name only without path - but in the script you are using /backup/${backup_filename} everywhere except within the trap command. I'm not sure, but it wont delete the file as it is in the wrong directory? Therefor changed var definition to be a full path similar to the $tmpfile

• running Mariadb backup with user password set as command line parameter might be insecure as it is visible to others, created Mariadb config file instead with all relevant parameter and just pass this as mariadb-dump default config (instead of ignoring all config files)

• Mariadb can be accessed via (bind mounted) Unix socket too - parsed this too if set and added to backup config file (https://github.com/joealcorn/mmonit/blob/b3e72f99d20273699ed433d14e9dbdbabd441c61/templates/etc/mmonit/server.xml#L182)

• on creating $tmpfile - the env var $prgname is not defined - or does it come somewhere from s6-overlay? (line 82)

• i'm not sure if its good to backup all databases here. On a shared MariaDB server the MMonit user has (or should have) access to its own database only. Shall the backup program run for the mmonit database only? Or "--all-databases"?