rework internal handling of strings to minimize temporary allocation; impacts rewrite() and rewrite_session() behavior in case of no rewriting; set LDAPURLDesc lud_scope to LDAP_SCOPE_DEFAULT if set to LDAP_SCOPE_BASE, so no trailing '??base' is added to referrals...

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