ITS#5408 part 6 - referrals.

ITS#5408 part 6 - referrals.
- Sending continuation references:
  Drop an unnecessary and wrong LDAP_SCOPE_BASE check.  The correct
  check would have been for DN == baseDN.
  Do not rewrite the scope, the caller already did.
- ldif_prepare_create(): Check that parent/newSuperior is not a referral.
- Rewrite ldif_back_rerferrals().  Better error handling.  Always pass
  refs through referral_rewrite().  As in back-bdb, apply manageDSAit
  only to the baseDN and do not use default_referral.