Configurable preference list for SSH host key types.

Configurable preference list for SSH host key types.

Now we actually have enough of them to worry about, and especially
since some of the types we support are approved by organisations that
people might make their own decisions about whether to trust, it seems
worth having a config list for host keys the same way we have one for
kex types and ciphers.

To make room for this, I've created an SSH > Host Keys config panel,
and moved the existing host-key related configuration (manually
specified fingerprints) into there from the Kex panel.