Remove a lot of pointless 'struct' keywords.

This is the commit that f3295e0fb _should_ have been. Yesterday I just
added some typedefs so that I didn't have to wear out my fingers
typing 'struct' in new code, but what I ought to have done is to move
all the typedefs into defs.h with the rest, and then go through
cleaning up the legacy 'struct's all through the existing code.

But I was mostly trying to concentrate on getting the test suite
finished, so I just did the minimum. Now it's time to come back and do
it better.

ssh1bpp.c

@@ -69,7 +69,7 @@ static void ssh1_bpp_free(BinaryPacketProtocol *bpp)
 }
 
 void ssh1_bpp_new_cipher(BinaryPacketProtocol *bpp,
-                         const struct ssh1_cipheralg *cipher,
+                         const ssh1_cipheralg *cipher,
                          const void *session_key)
 {
     struct ssh1_bpp_state *s;