Add a preference list for SSH-2 key exchange algorithms, on a new "Kex" panel

Add a preference list for SSH-2 key exchange algorithms, on a new "Kex" panel
(which will gain more content anon).

Retire BUG_SSH2_DH_GEX and add a backwards-compatibility wart, since we never
did find a way of automatically detecting this alleged server bug, and in any
case there was only ever one report (<3D91F3B5.7030309@inwind.it>, FWIW).

Also generalise askcipher() to a new askalg() (thus touching all the
front-ends).

I've made some attempt to document what SSH key exchange is and why you care,
but it could use some review for clarity (and outright lies).

git-svn-id: http://svn.tartarus.org/sgt/putty@5022 cda61777-01e9-0310-a592-d414129be87e