RENAME TABLE against a table with DATA/INDEX DIRECTORY overwrites
the file to which the symlink points.

This is security issue, because it is possible to create a table with
some name in some non-system database and set DATA/INDEX DIRECTORY
to mysql system database. Renaming this table to one of mysql system
tables (e.g. user, host) would overwrite the system table.

Return an error when the file to which the symlink points exist.