Support for selecting AES from the GUI. In the process, I've had to

introduce another layer of abstraction in SSH2 ciphers, such that a
single `logical cipher' (as desired by a user) can equate to more
than one `physical cipher'. This is because AES comes in several key
lengths (PuTTY will pick the highest supported by the remote end)
and several different SSH2-protocol-level names (aes*-cbc,
rijndael*-cbc, and an unofficial one rijndael-cbc@lysator.liu.se).

git-svn-id: svn://svn.tartarus.org/sgt/putty@967 cda61777-01e9-0310-a592-d414129be87e