Make prototype for new_prompts() consistent.

In commit b4c8fd9d8 which introduced the Seat trait, I got a bit
confused about the prototype of new_prompts(). Previously it took a
'Frontend *' parameter; I edited the call sites to pass a 'Seat *'
instead, but the actual function definition takes no parameters at all
- and rightly so, because the 'Frontend *' inside the prompts_t has
been removed and _not_ replaced with a 'Seat *', so the constructor
would have nothing to do with such a thing anyway.

But I wrote the function declaration in putty.h with '()' rather than
'(void)' (too much time spent in C++), and so the compiler never
spotted the mismatch.

Now new_prompts() is consistently nullary everywhere it appears: the
prototype in the header is a proper (void) one, and the call sites
have been modified to not pointlessly give it a Seat or null pointer.

cmdgen.c

@@ -906,7 +906,7 @@ int main(int argc, char **argv)
      * we have just generated a key.
      */
     if (!new_passphrase && (change_passphrase || keytype != NOKEYGEN)) {
-        prompts_t *p = new_prompts(NULL);
+        prompts_t *p = new_prompts();
         int ret;
 
         p->to_server = false;