frontend stuff moved into a database structure, essentially to allow overlays to be processed before database selection; passes all tests (ITS#3080)

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)

the lastmod command will fail.  To be set by backends
(e.g., back-ldap) which force the value of SLAP_DBFLAG_NOLASTMOD
database flag.

initial/final string settings from any string settings.

honor sizelimit requests on the overall count when pagedResults control is used; cleanup and more features

improve previous commit; now special limit on the number of total entries of a paged results can be set via the limits.  Need to decide whether the default should be unlimited or what

use "expand" instead of "regex" for group ACLs that allow substring expansion, preserving backwards compatibility; add sanity checks