Make dbEnv a private toy of back-bdb2.

servers/slapd/back-bdb2/add.c

servers/slapd/back-bdb2/alias.c

servers/slapd/back-bdb2/back-bdb2.h

servers/slapd/back-bdb2/bind.c

servers/slapd/back-bdb2/close.c

servers/slapd/back-bdb2/compare.c

servers/slapd/back-bdb2/delete.c

servers/slapd/back-bdb2/group.c

servers/slapd/back-bdb2/init.c

servers/slapd/back-bdb2/modify.c

servers/slapd/back-bdb2/modrdn.c

servers/slapd/back-bdb2/porter.c

servers/slapd/back-bdb2/proto-back-bdb2.h

servers/slapd/back-bdb2/search.c

servers/slapd/back-bdb2/startup.c

servers/slapd/back-bdb2/txn.c

servers/slapd/back-bdb2/txn.h