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

internal/cli/cli.go

@@ -1,7 +1,7 @@
 // Package 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.
+// runs before it starts the server, and, given no arguments, the
+// server, with the fx wiring.
 package cli
 
 import (
@@ -22,16 +22,21 @@ import (
 	"go.uber.org/fx/fxevent"
 )
 
+// usageExitCode is what Main returns for a command line it does not
+// understand, the usual exit code for a usage error.
+const usageExitCode = 2
+
 // Main runs netwatch-server with args, its command line after the
-// program's name, and returns the exit code; the subcommands write
-// their errors to stderr. Unless args are "check-cidr CIDR" or
-// "prepare-data-dir DATA_DIR", Main runs the server by calling serve
-// with appname and version, the name and version the server logs and
-// reports. main passes os.Stderr and RunServer; the tests pass a
-// buffer and a function that fails them, so that no test starts the
-// server. When serve is RunServer and the server fails to start, to
-// listen or to stop, fx's Run exits the process with status 1 and Main
-// never returns.
+// program's name, and returns the exit code. With no args, Main runs
+// the server by calling serve with appname and version, the name and
+// version the server logs and reports. With "check-cidr CIDR" or
+// "prepare-data-dir DATA_DIR" it runs that subcommand, which writes its
+// errors to stderr. Any other args get a one-line usage message on
+// stderr and exit code 2. main passes os.Stderr and RunServer; the
+// tests pass a buffer and a function of their own, so that no test
+// starts the server. When serve is RunServer and the server fails to
+// start, to listen or to stop, fx's Run exits the process with status 1
+// and Main never returns.
 func Main(
 	stderr io.Writer,
 	appname, version string,
@@ -53,6 +58,15 @@ func Main(
 		return prepareDataDir(stderr, "/data", args[1], "netwatch")
 	}
 
+	// Any other command line is refused, so that a mistyped one does
+	// not start the server.
+	if len(args) > 0 {
+		_, _ = fmt.Fprintln(stderr,
+			"usage: netwatch-server [check-cidr CIDR | prepare-data-dir DATA_DIR]")
+
+		return usageExitCode
+	}
+
 	serve(appname, version)
 
 	return 0