basic/strv: check printf arguments to strv_extendf()

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.