add support for recursive referrals (with simple loop detection) to slapo-chain (ITS#4070); add simple support for returning the error code instead of the original referral (ITS#4570)

prevents delay of client, removes dependency on slap_replog knowledge

Note about possible ITS#4158 breakage for SLAPI_CONN_DN

Don't pollute o_req_ndn after resetting (hyc please review)

specific backend for the DN (this was broken in the case that subordinate
and subsubordinate backends were glued)

(mirrors code in slapi_pblock.c)