ssh client known_hosts handling outstanding work

[mtrmac]

• [ ] Interactively prompt for known_hosts inclusion before adding an entry
• [ ] Either make handling of missing known_hosts and known_hosts that miss an entry for a specific machine consistent, or justify the differences
• [ ] Document the rationale for any compromises / deviations from the basic expectation that all ssh connections are secure, except for cases where the user makes an intentional decision to the contrary. That includes the precise semantics of InsecureIsMachineConnection.
• [ ] Examine how connection_golang.go and connection_native.go are / are not consistent (e.g. WRT interactive prompting methods and file descriptors used, or InsecureIsMachineConnection, if it becomes applicable to the native mode)
• [ ] Look into replacing InsecureIsMachineConnection with provisioning private keys into the machine, and using public keys to create the client connection.