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>

Cargo.toml

@@ -27,7 +27,13 @@ static_assertions = "1.1"
 
 # Network
 socket2 = { version = "0.6", features = ["all"] }
-nix = { version = "0.31", default-features = false, features = ["net", "fs"] }
+nix = { version = "0.31", default-features = false, features = [
+  "net",
+  "user",
+  "process",
+  "fs",
+  "signal",
+] }
 shadowsocks = { version = "1.24", features = ["aead-cipher-2022"] }
 
 # Serialization
@@ -65,8 +71,14 @@ hyper = { version = "1", features = ["server", "http1"] }
 hyper-util = { version = "0.1", features = ["tokio", "server-auto"] }
 http-body-util = "0.1"
 httpdate = "1.0"
-tokio-rustls = { version = "0.26", default-features = false, features = ["tls12"] }
-rustls = { version = "0.23", default-features = false, features = ["std", "tls12", "ring"] }
+tokio-rustls = { version = "0.26", default-features = false, features = [
+  "tls12",
+] }
+rustls = { version = "0.23", default-features = false, features = [
+  "std",
+  "tls12",
+  "ring",
+] }
 webpki-roots = "1.0"
 
 [dev-dependencies]