first step towards removing back-*/external.h

servers/slapd/back-bdb/init.c

servers/slapd/back-dnssrv/init.c

servers/slapd/back-ldap/init.c

servers/slapd/back-ldbm/init.c

servers/slapd/back-meta/init.c

servers/slapd/back-monitor/init.c

servers/slapd/back-null/null.c

servers/slapd/back-passwd/init.c

servers/slapd/back-perl/init.c

servers/slapd/back-relay/init.c

servers/slapd/back-shell/init.c

servers/slapd/back-sql/init.c