Add some host key dialogue boxes. These still look a little rough around

Add some host key dialogue boxes.  These still look a little rough around
the edges and need to have all their controls properly aligned and spaced
according to the HI guidelines.  Also, fix store_host_key() so that it
replaces a host key correctly when the host key has changed and the user
opts to update the cached one.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5280 cda61777-01e9-0310-a592-d414129be87e