winbind idmap plugin: support inferface version 6
    
    With Samba 4.7 the interface version of the idmap plugin was updated to
    6. The patch adds support for this new version but can be complied with
    the older version as well.
    
    A configure option is added to select the version, if no version is
    given configure tries to detect the version with the help of an internal
    Samba library libidmap-samba4.so.
    
    To make sure that always the right version is used configure will fail
    if Samba is used (--with-samba, default) and no version can be
    determined.
    
    Resolves https://pagure.io/SSSD/sssd/issue/3741
    
    Reviewed-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>