Yesterday's proxy enhancements also slightly nadgered the config

Yesterday's proxy enhancements also slightly nadgered the config
box, in that it started to expand under the weight of proxy options.
Now fixed, by folding the SOCKS version selector into the general
proxy type selector so there's one single 5- or 6-way radio button
set split over two lines. settings.c has of course grown a backwards
compatibility wart to deal with legacy config data.

git-svn-id: svn://svn.tartarus.org/sgt/putty@3168 cda61777-01e9-0310-a592-d414129be87e