Use be_match() in backend.c operational processing

values in order to test equivalence, because the overlay engine
may have reset the backend pointer to a temporary copy on the stack.

So, we test pointer equivalence of the BackendDB itself, then of
be_nsuffix -- this macro can be updated if necessary.

Added OpenLDAPaciSyntax based on ITS#3877 by Nikita Shulga
aci_mask() doesn't exploit the normalized value yet (next step)
The case #define SLAPD_ACI_ENABLED / #undef SLAP_DYNACL should
be removed