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/cli.rs

@@ -1,10 +1,13 @@
-//! CLI commands: --init (fire-and-forget setup)
+//! CLI commands: --init (fire-and-forget setup), daemon options
 
 use rand::RngExt;
 use std::fs;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+#[cfg(unix)]
+use crate::daemon::DaemonOptions;
+
 /// Options for the init command
 pub struct InitOptions {
     pub port: u16,
@@ -15,6 +18,64 @@ pub struct InitOptions {
     pub no_start: bool,
 }
 
+/// Parse daemon-related options from CLI args.
+#[cfg(unix)]
+pub fn parse_daemon_args(args: &[String]) -> DaemonOptions {
+    let mut opts = DaemonOptions::default();
+    let mut i = 0;
+
+    while i < args.len() {
+        match args[i].as_str() {
+            "--daemon" | "-d" => {
+                opts.daemonize = true;
+            }
+            "--foreground" | "-f" => {
+                opts.foreground = true;
+            }
+            "--pid-file" => {
+                i += 1;
+                if i < args.len() {
+                    opts.pid_file = Some(PathBuf::from(&args[i]));
+                }
+            }
+            s if s.starts_with("--pid-file=") => {
+                opts.pid_file = Some(PathBuf::from(s.trim_start_matches("--pid-file=")));
+            }
+            "--run-as-user" => {
+                i += 1;
+                if i < args.len() {
+                    opts.user = Some(args[i].clone());
+                }
+            }
+            s if s.starts_with("--run-as-user=") => {
+                opts.user = Some(s.trim_start_matches("--run-as-user=").to_string());
+            }
+            "--run-as-group" => {
+                i += 1;
+                if i < args.len() {
+                    opts.group = Some(args[i].clone());
+                }
+            }
+            s if s.starts_with("--run-as-group=") => {
+                opts.group = Some(s.trim_start_matches("--run-as-group=").to_string());
+            }
+            "--working-dir" => {
+                i += 1;
+                if i < args.len() {
+                    opts.working_dir = Some(PathBuf::from(&args[i]));
+                }
+            }
+            s if s.starts_with("--working-dir=") => {
+                opts.working_dir = Some(PathBuf::from(s.trim_start_matches("--working-dir=")));
+            }
+            _ => {}
+        }
+        i += 1;
+    }
+
+    opts
+}
+
 impl Default for InitOptions {
     fn default() -> Self {
         Self {