Make transform*, --info-command, and execute-silent cancellable

Users can press CTRL-C after 1 second to terminate the command.

Close #3883

src/util/util_test.go

@@ -137,8 +137,11 @@ func TestOnce(t *testing.T) {
 	if o() {
 		t.Error("Expected: false")
 	}
-	if o() {
-		t.Error("Expected: false")
+	if !o() {
+		t.Error("Expected: true")
+	}
+	if !o() {
+		t.Error("Expected: true")
 	}
 
 	o = Once(true)
@@ -148,6 +151,9 @@ func TestOnce(t *testing.T) {
 	if o() {
 		t.Error("Expected: false")
 	}
+	if o() {
+		t.Error("Expected: false")
+	}
 }
 
 func TestRunesWidth(t *testing.T) {