Restore the ability to write out PPK v2.

This commit adds the capability in principle to ppk_save_sb, by adding
a fmt_version field in the save parameters structure. As yet it's not
connected up to any user interface in PuTTYgen, but I think I'll need
to, because currently there's no way at all to convert PPK v3 back to
v2, and surely people will need to interoperate with older
installations of PuTTY, or with other PPK-consuming software.

ssh.h

@@ -1235,6 +1235,11 @@ int rsa1_load_f(const Filename *filename, RSAKey *key,
                 const char *passphrase, const char **errorstr);
 
 typedef struct ppk_save_parameters {
+    unsigned fmt_version;              /* currently 2 or 3 */
+
+    /*
+     * Parameters for fmt_version == 3
+     */
     Argon2Flavour argon2_flavour;
     uint32_t argon2_mem;               /* in Kb */
     bool argon2_passes_auto;