rework static backend initialization

servers/slapd/back-bdb/add.c

servers/slapd/back-bdb/bind.c

servers/slapd/back-bdb/compare.c

servers/slapd/back-bdb/config.c

servers/slapd/back-bdb/ctxcsn.c

servers/slapd/back-bdb/delete.c

servers/slapd/back-bdb/extended.c

servers/slapd/back-bdb/external.h

servers/slapd/back-bdb/id2entry.c

servers/slapd/back-bdb/init.c

servers/slapd/back-bdb/modify.c

servers/slapd/back-bdb/modrdn.c

servers/slapd/back-bdb/operational.c

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

servers/slapd/back-bdb/referral.c

servers/slapd/back-bdb/search.c

servers/slapd/back-bdb/tools.c

servers/slapd/back-bdb/trans.c

servers/slapd/back-dnssrv/bind.c

servers/slapd/back-dnssrv/compare.c