Get rid of the fixed-size 'hostname' buffer in every port-forwarded

connection, and replace it with sensible dynamically allocated
storage. While I'm at it, get rid of the disgusting dual use between
storing an actual hostname and storing an incoming SOCKS request; we
now have a separate pointer variable for each.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9903 cda61777-01e9-0310-a592-d414129be87e