MDEV-29050 mariabackup issues error messages during InnoDB tablespaces export on partial backup preparing

The solution is to suppress error messages for missing tablespaces if
mariabackup is launched with "--prepare --export" options.

"mariabackup --prepare --export" invokes itself with --mysqld parameter.
If the parameter is set, then it starts server to feed "FLUSH TABLES ...
FOR EXPORT;" queries for exported tablespaces. This is "normal" server
start, that's why new srv_operation value is introduced.

Reviewed by Marko Makela.