Support argv-based vLLM startup in `vllm-wrapper` (#979)

Improve vllm-wrapper support for native vLLM installations and systemd-managed vLLM processes.

It introduces three related features:

    Passing the vLLM startup command as separate arguments after --.
    Forwarding logs from a systemd user unit through the wrapper with --journal-unit.
    Terminating the active proxy after a successful sleep request with --stop-pid.