Add and fix copyright headers

Don't attach them to the package.

Author: rburchell

cmd/multibuild/integration_test.go

@@ -1,3 +1,7 @@
+// Copyright 2025 Robin Burchell. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package main
 
 import (

cmd/multibuild/options.go

@@ -1,6 +1,7 @@
 // Copyright 2025 Robin Burchell. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package main
 
 import (

cmd/multibuild/options_test.go

@@ -1,6 +1,7 @@
 // Copyright 2025 Robin Burchell. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package main
 
 import (

cmd/multibuild/utils.go

@@ -1,6 +1,7 @@
 // Copyright 2025 Robin Burchell. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package main
 
 import (