Allow retrying SSH client auth.

If you initially try private key auth for example, allow retrying
with password auth and vice-versa.  This is needed for reasonable
fallback from private-key auth since we don't know in advance
which will work, and we don't want to disconnect an reconnect.

This takes the guardrails of adding a password.  Previously, it was
ensured that a password was always added immediately after a username.
This is to ensure when using the server for example and stuffing a
list full of all the user/pw pairs, each pw goes with a user.

Instead of returning an error, this now allows adding passwords
without a corrsponding user that won't be used, and there will be
no indication of why.

Death Star mode for this feature is better than not having fallback
in my opinion.