(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).
[originally from svn r5022]