util: move string_in_list to util_ext
    
    To be able to include string_in_list() without additional
    dependencies it is moved into a separate file.
    
    Related to https://pagure.io/SSSD/sssd/issue/3050
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>