Use bi_extra for back-monitor entry points. Backends should never
directly reference each other's symbols.

leave monitor calls in place, hide contents; in the long run, enable monitor and build as static by default

active. Just search to the end of the table, tracking the max isn't worth
the lock overhead.

rework slap_client_connect() and slap_discover_feature() accordingly

- 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.