macOS SSH: match password prompts to the login account behind ProxyCommand and HostKeyAlias #274

Follow-up from the review of #273 (fix for #270). Both are pre-existing and low severity.

1. Bare Password: prompts count as the login account. SSHAttempt.isLogin(nil) is true, so an account-less password prompt is offered the destination's saved password and may be saved as it. OpenSSH names the account in password prompts, and in keyboard-interactive prompts ((user@host) Password:), so this needs a non-OpenSSH tool or an old ssh in a ProxyCommand. Option: treat a nil account as the login account only when ssh -G reports no proxyjump/proxycommand.
2. HostKeyAlias hosts can't save a password. ssh puts the HostKeyAlias, not the HostName, in the password prompt, but SSHHost.configuration() builds the login account from user and hostname. The destination's own prompt doesn't match, so since fix(macos): never save a jump host's SSH password for the destination #273 the sheet doesn't offer Save in Keychain (before it, a saved password was never used for that prompt either). Option: use hostkeyalias from ssh -G when it's set.