back-sch.c: search users and groups through NSSWITCH
    
    Schema-compat plugin can be configured to serve users and groups through
    the plugin configuration entry in directory server:
    
    schema-compat-lookup-nsswitch: <user|group>
    schema-compat-nsswitch-min-id: <value>
    
    Separate trees should be configured to look up users and groups.  If
    minimal id value is missing, it will default to 1000.