Clarify mdb_dbi_open doc

The concurrency restriction is on threads within a single process.
Multiple processes can use mdb_dbi_open without affecting each other.
make clean: remove *.lo
Fix MDB_APPEND doc

We actually compare the new key against the last key, to prevent
corruption.
Merge remote-tracking branch 'origin/mdb.RE/0.9' into OPENLDAP_REL_ENG_2_4
ITS#8006 Use rootdn consistently on internal ops
ITS#8006