rework subsystem initialization (can be added dynamically...)

servers/slapd/back-monitor/back-monitor.h

servers/slapd/back-monitor/backend.c

servers/slapd/back-monitor/conn.c

servers/slapd/back-monitor/database.c

servers/slapd/back-monitor/entry.c

servers/slapd/back-monitor/init.c

servers/slapd/back-monitor/listener.c

servers/slapd/back-monitor/log.c

servers/slapd/back-monitor/operation.c

servers/slapd/back-monitor/overlay.c

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

servers/slapd/back-monitor/rww.c

servers/slapd/back-monitor/sent.c

servers/slapd/back-monitor/thread.c

servers/slapd/back-monitor/time.c