CLI `--help` and `--version` are handled correctly

Since we introduced our own parsing function for command line arguments, we have to make sure that clap's behaviour is handled correctly.
Clap's `get_matches_from_safe` returns an error of a certain kind, of which `ErrorKind::HelpDisplayed` and `ErrorKind::VersionDisplayed ` have to be handled to properly print the help/version and exit the program.
The clap error includes the message, so we print help/version in main now and ensure the program exits with `0` afterwards.

swap/src/bin/swap.rs

@@ -20,6 +20,8 @@ use std::future::Future;
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::time::Duration;
+use structopt::clap;
+use structopt::clap::ErrorKind;
 use swap::bitcoin::TxLock;
 use swap::cli::command::{parse_args_and_apply_defaults, Arguments, Command};
 use swap::database::Database;
@@ -44,7 +46,23 @@ async fn main() -> Result<()> {
         data_dir,
         debug,
         cmd,
-    } = parse_args_and_apply_defaults(env::args_os())?;
+    } = match parse_args_and_apply_defaults(env::args_os()) {
+        Ok(args) => args,
+        Err(e) => {
+            if let Some(clap_err) = e.downcast_ref::<clap::Error>() {
+                match clap_err.kind {
+                    ErrorKind::HelpDisplayed | ErrorKind::VersionDisplayed => {
+                        println!("{}", clap_err.message);
+                        std::process::exit(0);
+                    }
+                    _ => {
+                        bail!(e);
+                    }
+                }
+            }
+            bail!(e);
+        }
+    };
 
     match cmd {
         Command::BuyXmr {