Idiot me! store_host_key() was blindly _appending_ new host keys to

Idiot me! store_host_key() was blindly _appending_ new host keys to
the end of the host key file. This is perfectly all right if a host
key never changes, but it's completely useless if you need to
replace an existing entry. This version should do better.

git-svn-id: http://svn.tartarus.org/sgt/putty@3719 cda61777-01e9-0310-a592-d414129be87e