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>
Alexey