Cleanup unknown config directive handling.

servers/slapd/back-bdb/config.c

servers/slapd/back-dnssrv/config.c

servers/slapd/back-ldap/config.c

servers/slapd/back-ldbm/config.c

servers/slapd/back-meta/config.c

servers/slapd/back-monitor/init.c

servers/slapd/back-null/null.c

servers/slapd/back-passwd/config.c

servers/slapd/back-perl/config.c

servers/slapd/back-shell/config.c

servers/slapd/back-sql/config.c

servers/slapd/backover.c

servers/slapd/config.c

servers/slapd/overlays/dyngroup.c

servers/slapd/overlays/rwm.c

servers/slapd/slap.h