basic/strv: check printf arguments to strv_extendf()
    
    The second argument to _printf_() specifies where the arguments start. We need to
    use 0 in two cases: when the args in a va_list and can't be checked, and with journald
    logging functions which accept multiple format strings with multiple argument sets,
    which the _printf_ checker does not understand. But strv_extendf() can be checked.
    
    (cherry picked from commit 400102ec91aa3404848a04f49a43d49e1a181708)