Add Unix daemon mode with PID file and privilege dropping

Implement core daemon infrastructure for running telemt as a background
  service on Unix platforms (Linux, FreeBSD, etc.):

  - Add src/daemon module with classic double-fork daemonization
  - Implement flock-based PID file management to prevent duplicate instances
  - Add privilege dropping (setuid/setgid) after socket binding
  - New CLI flags: --daemon, --foreground, --pid-file, --run-as-user,
    --run-as-group, --working-dir

  Daemonization occurs before tokio runtime starts to ensure clean fork.
  PID file uses exclusive locking to detect already-running instances.
  Privilege dropping happens after bind_listeners() to allow binding
  to privileged ports (< 1024) before switching to unprivileged user.

Signed-off-by: Vladimir Krivopalov <argenet@yandex.ru>

src/main.rs

@@ -4,6 +4,8 @@ mod api;
 mod cli;
 mod config;
 mod crypto;
+#[cfg(unix)]
+mod daemon;
 mod error;
 mod ip_tracker;
 #[cfg(test)]
@@ -27,7 +29,42 @@ mod tls_front;
 mod transport;
 mod util;
 
-#[tokio::main]
-async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
-    maestro::run().await
+fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
+    // On Unix, handle daemonization before starting tokio runtime
+    #[cfg(unix)]
+    {
+        let args: Vec<String> = std::env::args().skip(1).collect();
+        let daemon_opts = cli::parse_daemon_args(&args);
+
+        // Daemonize if requested (must happen before tokio runtime starts)
+        if daemon_opts.should_daemonize() {
+            match daemon::daemonize(daemon_opts.working_dir.as_deref()) {
+                Ok(daemon::DaemonizeResult::Parent) => {
+                    // Parent process exits successfully
+                    std::process::exit(0);
+                }
+                Ok(daemon::DaemonizeResult::Child) => {
+                    // Continue as daemon child
+                }
+                Err(e) => {
+                    eprintln!("[telemt] Daemonization failed: {}", e);
+                    std::process::exit(1);
+                }
+            }
+        }
+
+        // Now start tokio runtime and run the server
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()?
+            .block_on(maestro::run_with_daemon(daemon_opts))
+    }
+
+    #[cfg(not(unix))]
+    {
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()?
+            .block_on(maestro::run())
+    }
 }