streamline back-ldap/slapo-chain configuration; add the possibility to cache non-configured, anonymous referral URIs

fix dangling resources issue in slapd-ldap; completely rework slapo-chain to fix the resource leak/concurrency issue; add support for multiple well-known URIs to set credentials for, and deal with unknown URIs anonymously; similar reworking and cleanup for slapd-meta

extraneous information

check reponse tags in ldapmodify; other functions rely on client library, which might need further work.  Also, remove or silence all deprecated API calls

reduce library link tries.

not yet used