all: use strs.CutPrefix and strs.CutSuffix more

Updates places where we use HasPrefix + TrimPrefix to use the combined
function.

Updates #5309

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tailscale/cli/file.go

@@ -30,6 +30,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/quarantine"
+	"tailscale.com/util/strs"
 	"tailscale.com/version"
 )
 
@@ -77,10 +78,10 @@ func runCp(ctx context.Context, args []string) error {
 		return errors.New("usage: tailscale file cp <files...> <target>:")
 	}
 	files, target := args[:len(args)-1], args[len(args)-1]
-	if !strings.HasSuffix(target, ":") {
+	target, ok := strs.CutSuffix(target, ":")
+	if !ok {
 		return fmt.Errorf("final argument to 'tailscale file cp' must end in colon")
 	}
-	target = strings.TrimSuffix(target, ":")
 	hadBrackets := false
 	if strings.HasPrefix(target, "[") && strings.HasSuffix(target, "]") {
 		hadBrackets = true