cln-plugin: Handle --help invocations better

We now have ternary outcomes for `Builder.configure()` and
`Builder.start()`:

 - Ok(Some(p)) means we were configured correctly, and can continue
   with our work normally
 - Ok(None) means that `lightningd` was invoked with `--help`, we
   weren't configured (which is not an error since the `lightningd` just
   implicitly told us to shut down) and user code should clean up and
   exit as well
 - Err(e) something went wrong, user code may report an error and exit.

plugins/grpc-plugin/src/main.rs

@@ -28,19 +28,25 @@ async fn main() -> Result<()> {
         ca_cert,
     };
 
-    let plugin = Builder::new(state.clone(), tokio::io::stdin(), tokio::io::stdout())
+    let plugin = match Builder::new(state.clone(), tokio::io::stdin(), tokio::io::stdout())
         .option(options::ConfigOption::new(
             "grpc-port",
             options::Value::Integer(-1),
             "Which port should the grpc plugin listen for incoming connections?",
         ))
         .configure()
-        .await?;
+        .await?
+    {
+        Some(p) => p,
+        None => return Ok(()),
+    };
 
     let bind_port = match plugin.option("grpc-port") {
         Some(options::Value::Integer(-1)) => {
             log::info!("`grpc-port` option is not configured, exiting.");
-            plugin.disable("`grpc-port` option is not configured.").await?;
+            plugin
+                .disable("`grpc-port` option is not configured.")
+                .await?;
             return Ok(());
         }
         Some(options::Value::Integer(i)) => i,