Initial support for host certificates.

Now we offer the OpenSSH certificate key types in our KEXINIT host key
algorithm list, so that if the server has a certificate, they can send
it to us.

There's a new storage.h abstraction for representing a list of trusted
host CAs, and which ones are trusted to certify hosts for what
domains. This is stored outside the normal saved session data, because
the whole point of host certificates is to avoid per-host faffing.

Configuring this set of trusted CAs is done via a new GUI dialog box,
separate from the main PuTTY config box (because it modifies a single
set of settings across all saved sessions), which you can launch by
clicking a button in the 'Host keys' pane. The GUI is pretty crude for
the moment, and very much at a 'just about usable' stage right now. It
will want some polishing.

If we have no CA configured that matches the hostname, we don't offer
to receive certified host keys in the first place. So for existing
users who haven't set any of this up yet, nothing will immediately
change.

Currently, if we do offer to receive certified host keys and the
server presents one signed by a CA we don't trust, PuTTY will bomb out
unconditionally with an error, instead of offering a confirmation box.
That's an unfinished part which I plan to fix before this goes into a
release.

putty.h

@@ -2586,6 +2586,11 @@ void conf_fontsel_handler(union control *ctrl, dlgparam *dlg,
 void setup_config_box(struct controlbox *b, bool midsession,
                       int protocol, int protcfginfo);
 
+void setup_ca_config_box(struct controlbox *b);
+
+/* Platforms provide this to be called from config.c */
+void show_ca_config_box(dlgparam *dlg);
+
 /* Visible outside config.c so that platforms can use it to recognise
  * the proxy type control */
 void proxy_type_handler(union control *ctrl, dlgparam *dlg,