New creates a new Upgrader.

DefaultUpgradeTimeout is the duration before the Upgrader kills the new process if no readiness notification was received.

Fds holds all file descriptors inherited from the parent process.

Options control the behaviour of the Upgrader.

Upgrader handles zero downtime upgrades and passing files between processes.

Conn can be shared between processes.

Listener can be shared between processes.

PacketConn can be shared between processes.