slightly rework user/operational attributes handling (including fixing a bug in the logic of the previous change to backend_operational()); cleanup; more improvements to slapo-rwm and back-relay

revert previous change; anticipate backend_operational before overlays callbacks (thanks, Howard); rework overlays cycle to allow callbacks to free themselves (thanks Howard)

(and plug leaks)

misc other cleanup

(back-sql needs to be updated)

result to client.

(slapi_x_XXX is still reserved for exported functions that are not part
of the SLAPI specification)