* 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
Initial sd_notify support was added in #3963, but that sent signals from
both cmdRun and cmdReload. This approach has two drawbacks:
- Reloads initiated via the API do not send signals.
- The signals are sent from different processes, which requires the
`NotifyAccess=exec` directive in the unit file.
This change moves the NotifyReloading and NotifyReadiness invocations to
Load, which address both of those drawbacks. It also adds a
complimentary NotifyStopping method which is invoked from handleStop.
All the notify methods are defined in a notify package to avoid an
import loop.