libldap_r

It seems the dichotomy between libldap and libldap_r is a relic from the bad old days of dcethreads / cmathreads when linking a threaded library into an otherwise non-threaded program would cause all sorts of strange and wonderful failures. Unless anyone knows of any current platform where this is unsafe, I think it's time we dropped this distinction, and just use libldap_r (until we get to writing a completely new C API).

libldap_r is still missing some thread-specific features though - we should wrap all library initialization in a pthread_once() call, and we should be using thread-specific data for the LDAP* errno value.