Support preseeding of tasksel/first.

tasksel-debconf

@@ -8,7 +8,9 @@ defaults="$3"
 question=$4
 db_settitle tasksel/title
 db_subst $question CHOICES "$choices"
-db_set $question "$defaults"
+if [ "x$defaults" != "x-" ]; then
+	db_set $question "$defaults"
+fi
 db_input high $question || true
 db_go || true
 db_fset $question seen false

tasksel.pl

@@ -413,7 +413,8 @@ sub main {
 		my $tmpfile=`tempfile`;
 		chomp $tmpfile;
 		system($debconf_helper, $tmpfile, task_to_debconf(@list),
-			task_to_debconf(@default), $question);
+			($options{"new-install"} ? "-" : task_to_debconf(@default)),
+			$question);
 		open(IN, "<$tmpfile");
 		my $ret=<IN>;
 		chomp $ret;