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 :-)

[originally from svn r8033]