more work on porting command parsing to Go

tools/cmd/at/main.go

@@ -9,7 +9,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"strconv"
 	"strings"
 	"time"
 

tools/cmd/at/scroll_window.go

@@ -8,8 +8,8 @@ import (
 	"strings"
 )
 
-func parse_scroll_amount(amt string) ([2]interface{}, error) {
-	var ans [2]interface{}
+func parse_scroll_amount(amt string) ([]interface{}, error) {
+	var ans = make([]interface{}, 2)
 	if amt == "start" || amt == "end" {
 		ans[0] = amt
 		ans[1] = nil

tools/cmd/at/set_background_opacity.go

@@ -0,0 +1,19 @@
+// License: GPLv3 Copyright: 2022, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package at
+
+import "strconv"
+
+func parse_opacity(arg string) (float64, error) {
+	ans, err := strconv.ParseFloat(arg, 64)
+	if err != nil {
+		return 0, nil
+	}
+	if ans < 0.1 {
+		ans = 0.1
+	}
+	if ans > 1 {
+		ans = 1
+	}
+	return ans, nil
+}