- in case a value is incorrectly empty (technically, it's null because
  that's how it's returned by the stringification function), make enough
  room to add the error string "(null)"
- in case a value is legitimately empty (see above), use it accordingly;
  for example, a filter "(entryDN=)" is valid, so it should be possible
  to stringify it like that.

more cleanup; add the frontend to the set of monitored databases; handle exceptional cases a bit more smoothly

to clear out.

- in case we appended an entry to a yet unknown parent, we don't know
  the full DN, so unregistering needs to be possible based on the
  available data;
- complete unregister calls so that not-yet-registered stuff can be
  removed from limbo

defer handling of loglevel listing, to let modules register custom subsystems (ITS#4666).  Note: in case of any error in between, logging could not occur

- moved directory path publication from back-monitor/database.c
- added cleanup handler
- minor cleanup

  database entry in cn=monitor
- improve support for custom attribute/callback registration
  in back-monitor
- design API for registered antry/attr/callback removal in back-monitor

HEADS UP: now back-bdb requires back-monitor to be present, either static
or dynamically loaded __before__ back-bdb/back-hdb is initialized.
This works for sure if both back-bdb & back-monitor are static or dynamic
(provided back-monitor.la is loaded before back_bdb.la); it is not clear
what happens if back-bdb is static and back-monitor is dynamic.  In case,
either make back-bdb monitoring conditional, or hardwire back-monitor
much like back-config and back-ldif.