[sshd] Close SSH connections after 15 minutes of inactivity

[jleveque]

- What I did

Configure sshd to close all SSH connetions after 15 minutes of inactivity.

- How I did it

Set ClientAliveInterval to 900 (900 seconds = 15 minutes) and ClientAliveCountMax to 0
in /etc/ssh/sshd_config using augtool in build_debian.sh. In the process, I refactored the existing augtool command for sshd_config so as to add comments and empty lines to file for readability.

- How to verify it

1. Log into device via management port. Wait 15 minutes without sending a keystroke -- you should be automatically logged out.

[jleveque]

Retest vs please

[toreanderson]

This is an abuse of the OpenSSH ClientAlive* settings, which are not intended as a generic idle timeout, see https://bugzilla.mindrot.org/show_bug.cgi?id=3172#c6 and https://bugzilla.mindrot.org/show_bug.cgi?id=3362#c2.

The fact that it did with ClientAliveCountMax 0, is actually a bug that was fixed in OpenSSH 8.2, see https://bugzilla.mindrot.org/show_bug.cgi?id=2627#c2. So the desired behaviour will stop working.

Fortunately, I might add. This is a pretty annoying «feature» – getting thrown out every 15 minutes is infuriating. Even one is not actually sending any keystrokes to the SSH session this does not mean that it is «inactive». One might be trying to debug some issue by passively monitoring streaming log output from journalctl -f, for example.

This pull request is IMHO missing a clear rationale or a «Why I did it» section. This is not the default behaviour in OpenSSH nor in any other mainstream Linux distribution as far as I know. Why deviate?