ssh remoting: Daemonize server process properly by forking and closing stdio (#19544)

This fixes the `ssh` proxy process not being notified when the proxy
process dies. Turns out that the server would have stdout/stderr/stdin
connected to the grand-parent ssh process connected to it and as long as
the server kept running (even once it was daemonized into the
background) the grand-parent ssh process wouldn't exit.

That in turn meant that the Zed client wasn't notified when the proxy
process died.

Release Notes:

- N/A

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Co-authored-by: Bennet <bennet@zed.dev>