* 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
Matthew Holt