Detect invalid file permissions on existing SSH key pairs

[thomasjo]

When a user executes prepare-authentication.sh with a pre-existing key pair, we should detect if the permissions are invalid.
And if the permissions are invalid, we should inform the user in a "friendly" manner and help them fix the permission issue.

An alternative approach would be to fix the permission issue automatically, but I'm a bit reluctant about changing file permissions on existing files; they might be "invalid" for a reason.

For reference, the .ssh folder needs to be 0700 and any file within that folder should be 0600.