2255/51: no DN v empty DN v original DN

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

RFC 2255 is not explicit in regards to what DN is implied when
there is no DN part specified.  I believe the general
interpretation (excepting use in referrals) is that an empty
DN is implied.  I believe this should be explicit in 2255bis.

RFC 2251 referral processing describes DN defaulting when the
DN part is "not present".  I suggest that this be clarified as
well to say the defaulting occurs when the DN part is
empty (explicit or implied by 2255 defaulting).

Note that the 2251 defaulting of the DN (whether by "not present"
or "is empty") disallows certain types of referrals from being
generated.  Specifically, a server cannot refer a client to
a LDAP server AND specify the new DN as being the root of the
DIT.  I am not sure where such a referral would be useful, but
it is disallowed.  2251 should minimally state this restrict is
present.  If it is desirable to remove this restriction (which
I don't think it is), the specification would have to be modified
to disallow any DN defaulting to the original DN and require
servers to always specify the appropriate DN to be used in
continuing the operation.  I do not advocate such a change.

Kurt