consistently issue a
Note 1618 DATA DIRECTORY option ignored
Note 1618 INDEX DIRECTORY option ignored
in archive/csv/innodb/rocksdb whenever an option is ignored.
Note that csv doesn't say "INDEX DIRECTORY option ignored"
because it does not create index files at all anywhere.
Other engines don't say "INDEX DIRECTORY option ignored"
if the table has no indexes.
additionally InnoDB doesn't say that if INDEX DIRECTORY is
the same as DATA DIRECTORY, because in that case indexes are
technically stored in INDEX DIRECTORY.
collateral fix: use strmake to zero-terminate the string
