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.

putty.h

@@ -1197,6 +1197,11 @@ void pgp_fingerprints(void);
 int verify_ssh_host_key(void *frontend, char *host, int port,
                         const char *keytype, char *keystr, char *fingerprint,
                         void (*callback)(void *ctx, int result), void *ctx);
+/*
+ * have_ssh_host_key() just returns true if a key of that type is
+ * already chached and false otherwise.
+ */
+int have_ssh_host_key(const char *host, int port, const char *keytype);
 /*
  * askalg has the same set of return values as verify_ssh_host_key.
  */

ssh.c

@@ -6350,9 +6350,20 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
         if (!s->got_session_id) {
             /*
              * In the first key exchange, we list all the algorithms
-             * we're prepared to cope with.
+             * we're prepared to cope with, but prefer those algorithms
+	     * for which we have a host key for this host.
              */
 	    n = 0;
+            for (i = 0; i < lenof(hostkey_algs); i++) {
+		if (have_ssh_host_key(ssh->savedhost, ssh->savedport,
+				      hostkey_algs[i]->keytype)) {
+		    assert(n < MAXKEXLIST);
+		    s->kexlists[KEXLIST_HOSTKEY][n].name =
+			hostkey_algs[i]->name;
+		    s->kexlists[KEXLIST_HOSTKEY][n].u.hostkey = hostkey_algs[i];
+		    n++;
+		}
+	    }
             for (i = 0; i < lenof(hostkey_algs); i++) {
 	        assert(n < MAXKEXLIST);
 		s->kexlists[KEXLIST_HOSTKEY][n].name = hostkey_algs[i]->name;

unix/uxstore.c

@@ -589,6 +589,16 @@ int verify_host_key(const char *hostname, int port,
     return ret;
 }
 
+int have_ssh_host_key(const char *hostname, int port,
+		      const char *keytype)
+{
+    /*
+     * If we have a host key, verify_host_key will return 0 or 2.
+     * If we don't have one, it'll return 1.
+     */
+    return verify_host_key(hostname, port, keytype, "") != 1;
+}
+
 void store_host_key(const char *hostname, int port,
 		    const char *keytype, const char *key)
 {

windows/winstore.c

@@ -454,6 +454,16 @@ int verify_host_key(const char *hostname, int port,
 	return 0;		       /* key matched OK in registry */
 }
 
+int have_ssh_host_key(const char *hostname, int port,
+		      const char *keytype)
+{
+    /*
+     * If we have a host key, verify_host_key will return 0 or 2.
+     * If we don't have one, it'll return 1.
+     */
+    return verify_host_key(hostname, port, keytype, "") != 1;
+}
+
 void store_host_key(const char *hostname, int port,
 		    const char *keytype, const char *key)
 {