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
