feat: add optional variadic args to all commands

Allow callers to pass additional systemctl flags (e.g. --no-block,
--force) via variadic string args on every exported function.

This is backward-compatible: existing callers without extra args
continue to work unchanged.

Introduces a prepareArgs helper to centralize argument construction,
replacing the duplicated args/UserMode pattern across all functions.

Closes #2

util.go

@@ -55,6 +55,20 @@ func execute(ctx context.Context, args []string) (string, string, int, error) {
 	return output, warnings, code, err
 }
 
+// prepareArgs builds the systemctl command arguments from a base command,
+// options, and any additional arguments the caller wants to pass through.
+func prepareArgs(base string, opts Options, extra ...string) []string {
+	args := make([]string, 0, 2+len(extra))
+	args = append(args, base)
+	if opts.UserMode {
+		args = append(args, "--user")
+	} else {
+		args = append(args, "--system")
+	}
+	args = append(args, extra...)
+	return args
+}
+
 func filterErr(stderr string) error {
 	switch {
 	case strings.Contains(stderr, `does not exist`):