Bug description: If a replication is configured against a backend
before initializing the backend with a suffix entry, an RUV entry
is inserted first with the entryid 1. The RUV entry's entryrdn is
added to the entryrdn index with a suffix entry which is a parent
entry of the RUV entry having a temporary entryid 0, which was to
be replaced with the real entryid when the real suffix entry is
added. But the replacement code was not executed.
Fix description: When a real suffix is added to the entryrdn index,
it returns DB_KEYEXIST, which used to be ignored by resetting 0
(== SUCCESS). This patch returns DB_KEYEXIST to the caller and let
_entryrdn_insert_key use the info to replace the temporary entryid
with the real one. The error code is ignored by the other callers.