caddycmd: Add `--keep-backup` to upgrade commands (#4387)

* caddycmd: Add `--skip-cleanup` to upgrade commands

This is a partial fix for https://github.com/caddyserver/caddy/issues/4057, making it possible to retain the old build of Caddy, in case something went wrong.

* caddycmd: Fix duplicate error message

The error message "download succeeded, but unable to execute" was repeated, because it was both in the `listModules`/`showVersion` functions and in the calling `upgradeBuild` function. Oversight when this was refactored.

* caddycmd: Implement fix for performing cleanup on Windows

Without this, the cleanup operation would fail with an error message like this:

upgrade: download succeeded, but unable to clean up backup binary: remove C:\caddy\caddy.exe.tmp: Access is denied.

* caddycmd: Rename to `--keep-backup`, simplify build constraints

1 files changed, 30 insertions, 0 deletions

diff --git a/cmd/removebinary.go b/cmd/removebinary.go
new file mode 100644
index 000000000..adef6b104
--- /dev/null
+++ b/cmd/removebinary.go
@@ -0,0 +1,30 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !windows
+// +build !windows
+
+package caddycmd
+
+import (
+	"os"
+)
+
+// removeCaddyBinary removes the Caddy binary at the given path.
+//
+// On any non-Windows OS, this simply calls os.Remove, since they should
+// probably not exhibit any issue with processes deleting themselves.
+func removeCaddyBinary(path string) error {
+	return os.Remove(path)
+}