improve usability of global ovrlays for write operations; may need to anticipate SLAPI accordingly.  More work needed for add (overlays need to understand they're global and call slap_mods2entry() explicitly)

work out an issue with chk_referrals and back-relay/rwm; this looks like the best compromise; minor cleanup

- return those that failed as referrals
- fix handling of some operational attributes
- fix URI rebuilding (use default scope)
- make copies of o_req_dn/o_req_ndn in case other overlays muck with them