C sshnpd - stability enhancements and testing

[XavierChanth]

My main focus for this sprint is putting tests in place for C sshnpd. Getting it from MVP to beta status.

This will involve:

• Ensuring child processes are handled correctly (srv, ssh-keygen)
• Adding C sshnpd to end2end tests
• ephemeral ssh keys are properly cleaned up
• implementing npt_request for sshnpd

[XavierChanth]

Will open a new npt_request ticket, the rest has either been completed, or no longer needed because we removed the ephemeral tunnel