rsync and mariabackup SST ignored ignore_db_dirs: rsync copied and
deleted such directories like a normal schema dir, and mariabackup's
joiner wiped them during its pre-apply cleanup, failing SST outright
on a read-only mount. A directory name is also unvalidated by the
server, so it was spliced unchecked into an eval'd rsync filter.
Fix both SST scripts to read ignore_db_dirs and exclude those
directories, accepting only safe characters in the name.
Assisted-by: https://mariadb.org/governance/governance-ai-policy/
b6286fd276