netwatch-server refuses arguments it does not understand (closes #179)

cli.Main starts the server only when given no arguments. check-cidr
CIDR and prepare-data-dir DATA_DIR run as before. Any other command
line, such as a subcommand with no value or two values, or a misspelt
subcommand, gets a one-line usage message naming both forms on stderr
and exit code 2. It used to start the server, which then held the
port.

Model: opus-5-5

README.md

@@ -220,7 +220,8 @@ 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.
+  starts the server, and, given no arguments, the server, with the `fx` wiring.
+  It refuses any other arguments with a usage message and exit code 2.
 - **`config`**: Loads configuration from environment variables and config files
   via Viper.
 - **`handlers`**: HTTP request handlers for the API (health check, report