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.

unix/uxpgnt.c

@@ -681,7 +681,7 @@ void run_client(void)
 
                 if (key->ssh_version == 1) {
                     BinarySource src[1];
-                    struct RSAKey rkey;
+                    RSAKey rkey;
 
                     BinarySource_BARE_INIT(src, key->blob->u, key->blob->len);
                     memset(&rkey, 0, sizeof(rkey));