conf: improve --vt warning by allowing --seats=,all

If a seat is called "all" and we want to use it as single seat, we
currently have to pass "--seats=all," or "--seats=,all" etc. so it is not
detected as --seats=all.
However, our warning when --vt is used with it does not respect this.
Improve this warning to detect this case and correctly ignore it then.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

src/kmscon_conf.c

@@ -409,7 +409,7 @@ static int aftercheck_vt(struct conf_option *opt, int argc, char **argv,
 		return 0;
 
 	if (shl_string_list_is(conf->seats, "all") ||
-	    !conf->seats[0] || conf->seats[1])
+	    shl_string_list_count(conf->seats, true) != 1)
 		log_warning("you should use --vt only if --seats contains exactly one seat");
 
 	return 0;