Been meaning to do this for years: introduce a configuration option

Been meaning to do this for years: introduce a configuration option
to manually tweak the host name and port number under which the SSH
host key is read and written.

I've put it in the cross-platform Connection panel. Partly under the
flimsy pretext that other backends _can_ use it if they so wish (and
in fact it overrides the host name for title-bar purposes in all
network backends, though it has no other effect in anything but
SSH); but mostly because the SSH panel was too full already :-)

git-svn-id: svn://svn.tartarus.org/sgt/putty@8033 cda61777-01e9-0310-a592-d414129be87e