server: do not panic on --help command

This commit is including a simple check and suggest a help string
for the user when the command `--help` is used. Without this diff
applied the server will panic and the user will not have any clue
why.

     Running `target/debug/ldk-server --help`
thread 'main' panicked at ldk-server/src/main.rs:30:56:
Invalid configuration file.: Custom { kind: NotFound, error: "Failed to read config file '--help': No such file or directory (os error 2)" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[1]    750067 IOT instruction (core dumped)  cargo run --bin ldk-server -- --help

Signed-off-by: Vincenzo Palazzo <[email protected]>

ldk-server/src/main.rs

@@ -18,16 +18,29 @@ use ldk_node::config::Config;
 use std::path::Path;
 use std::sync::Arc;
 
+const USAGE_GUIDE: &str = r#"
+Usage: ldk-server [<option> ...] <config_path>
+
+Options
+    -h | --help        Print help
+"#;
+
 fn main() {
 	let args: Vec<String> = std::env::args().collect();
 
 	if args.len() < 2 {
-		eprintln!("Usage: {} config_path", args[0]);
+		eprintln!("{USAGE_GUIDE}");
 		std::process::exit(-1);
 	}
 
+	let arg = args[1].as_str();
+	if arg == "-h" || arg == "--help" {
+		println!("{}", USAGE_GUIDE);
+		std::process::exit(0);
+	}
+
 	let mut ldk_node_config = Config::default();
-	let config_file = load_config(Path::new(&args[1])).expect("Invalid configuration file.");
+	let config_file = load_config(Path::new(arg)).expect("Invalid configuration file.");
 
 	ldk_node_config.log_level = LogLevel::Trace;
 	ldk_node_config.storage_dir_path = config_file.storage_dir_path;