Exit cleanly on finished. Override auth and host keys.

[bulldozer2003]

• Have early-ssh exit with a status of 0 only if "finished" was typed in the SSH session. This potentially allows the init process to tell if early-ssh was used, times out, or exited from the local terminal.
• Add override for authorized keys file.
• Add override for directory containing dropbear host keys.
• Support edcsa, ed25519, and future host key types.