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.

(cherry picked from commit d183484742)

putty.h

@@ -667,7 +667,7 @@ typedef struct {
     void *data;         /* slot for housekeeping data, managed by
                          * seat_get_userpass_input(); initially NULL */
 } prompts_t;
-prompts_t *new_prompts();
+prompts_t *new_prompts(void);
 void add_prompt(prompts_t *p, char *promptstr, bool echo);
 void prompt_set_result(prompt_t *pr, const char *newstr);
 char *prompt_get_result(prompt_t *pr);