LDAP: Detect the presence of POSIX attributes
    
    When the schema is set to AD and ID mapping is not used, there is a one-time
    check ran when searching for users to detect the presence of POSIX
    attributes in LDAP. If this check fails, the search fails as if no entry
    was found and returns a special error code.
    
    The sdap_server_opts structure is filled every time a client connects to
    a server so the posix check boolean is reset to false again on connecting
    to the server.
    
    It might be better to move the check to where the rootDSE is retrieved,
    but the check depends on several features that are not known to the code
    that retrieves the rootDSE (or the connection code for example) such as what
    the attribute mappings are or the authentication method that should be used.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>