A connection is opened using nginx framework and then fed to OpenLDAP using
ldap_init_fd() call with custom SockBuf IO handlers. When some credentials
need to be validated, ngx_http_auth_ldap_authenticate() is called multiple
times, returning NGX_AGAIN each time the process is waiting for the LDAP
server to reply.
(Note: This only an initial implementation and as such, it is of course
very buggy and limited. Further work is needed to make it useful.)