SSH: Implement and make use of RFC 4256.

There is now at least one Linux distribution (openSUSE 12.3) that
disables the "password" authentication method in its default
sshd_config, while others allow it, but disable "keyboard-interactive".
This patch tackles the problem as follows:
    1) Implement RFC 4256 ("keyboard-interactive") and make this method
available in the API.
    2) In addition, the API offers to try both password-based methods
one after the other, until one has succeeded or all have failed.
    3) Dialogs continue to offer just the choice between "Password" and
"Key", as to not confuse users. Internally, "Password" uses the feature
described in 2).

Task-number: QTCREATORBUG-9568
Change-Id: Ic81bd5d2dc4b1332ea1a8be938c19811c21a9087
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>