Add have_ssh_host_key() and use it to influence algorithm selection.

Add have_ssh_host_key() and use it to influence algorithm selection.

The general plan is that if PuTTY knows a host key for a server, it
should preferentially ask for the same type of key so that there's some
chance of actually getting the same key again.  This should mean that
when a server (or PuTTY) adds a new host key type, PuTTY doesn't
gratuitously switch to that key type and then warn the user about an
unrecognised key.