Turn SSH-2 ciphers into a classoid.

This is more or less the same job as the SSH-1 case, only more
extensive, because we have a wider range of ciphers.

I'm a bit disappointed about the AES case, in particular, because I
feel as if it ought to have been possible to arrange to combine this
layer of vtable dispatch with the subsidiary one that selects between
hardware and software implementations of the underlying cipher. I may
come back later and have another try at that, in fact.

import.c

@@ -549,7 +549,7 @@ struct ssh2_userkey *openssh_pem_read(const Filename *filename,
             des3_decrypt_pubkey_ossh(keybuf, key->iv,
                                      key->keyblob->u, key->keyblob->len);
         else {
-            void *ctx;
+            AESContext *ctx;
             assert(key->encryption == OP_E_AES);
             ctx = aes_make_context();
             aes128_key(ctx, keybuf);