main() is one call into internal/cli (closes #160)

cmd/netwatch-server/main.go parsed the command line, ran the check-cidr
and prepare-data-dir subcommands and built the fx application itself;
the Go style guide puts all of that in internal/ or pkg/. The new
package internal/cli does it, with the same arguments, messages and
exit codes, and main() is now os.Exit(cli.Main(Appname, Version,
os.Args[1:])). Appname and Version stay in main, where the build sets
Version. Each subcommand writes its error to a writer it is given, and
prepare-data-dir also takes the volume and the user's name, so the new
unit tests give it a temporary directory and the user they run as.

Model: opus-5-5

README.md

@@ -207,6 +207,9 @@ stops. Its routes:
 It is an `fx`-wired Go application: `cmd/netwatch-server/` starts it, and its
 packages are in `internal/`:
 
+- **`cli`**: Runs netwatch-server from its command line: the `check-cidr` and
+  `prepare-data-dir` subcommands, which `bin/entrypoint.sh` runs before it
+  starts the server, and otherwise the server, with the `fx` wiring.
 - **`config`**: Loads configuration from environment variables and config files
   via Viper.
 - **`handlers`**: HTTP request handlers for the API (health check, report