Make ssh_key and ssh{2,1}_cipher into structs.

In commit 884a7df94 I claimed that all my trait-like vtable systems
now had the generic object type being a struct rather than a bare
vtable pointer (e.g. instead of 'Socket' being a typedef for a pointer
to a const Socket_vtable, it's a typedef for a struct _containing_ a
vtable pointer).

In fact, I missed a few. This commit converts ssh_key, ssh2_cipher and
ssh1_cipher into the same form as the rest.

sshrsag.c

@@ -14,7 +14,7 @@ int rsa_generate(struct RSAKey *key, int bits, progfn_t pfn,
     Bignum pm1, qm1, phi_n;
     unsigned pfirst, qfirst;
 
-    key->sshk = &ssh_rsa;
+    key->sshk.vt = &ssh_rsa;
 
     /*
      * Set up the phase limits for the progress report. We do this