(ITS#9016) cn=config should fail on EMIT if target directory not empty

[ondra@openldap.org]

Full_Name: Ondrej Kuznik
Version: re24/master
OS: 
URL: 
Submission from: (NULL) (82.10.24.68)


Running slapd/slaptest to convert a slapd.conf to cn=config will happily reuse
the target directory.

However if there was some existing configuration present already, slaptest
reports a successful conversion, even though the resulting directory most likely
won't represent the slapd.conf used - existing .ldif files won't be replaced or
removed. Depending on the differences between slapd.conf and slapd.d in place,
the result might not even be a useable cn=config anymore.

Since it's not feasible to clean up properly, it should just return an error
instead?