Use strbuf to store results in prompts_t.

UBsan pointed out another memcpy from NULL (again with length 0) in
the prompts_t system. When I looked at it, I realised that firstly
prompt_ensure_result_size was an early not-so-good implementation of
sgrowarray_nm that would benefit from being replaced with a call to
the real one, and secondly, the whole system for storing prompt
results should really have been replaced with strbufs with the no-move
option, because that's doing all the same jobs better.

So, now each prompt_t holds a strbuf in place of its previous manually
managed string. prompt_ensure_result_size is gone (the console
prompt-reading functions use strbuf_append, and everything else just
adds to the strbuf in the usual marshal.c way). New functions exist to
retrieve a prompt_t's result, either by reference or copied.

unix/uxpgnt.c

@@ -338,7 +338,7 @@ static char *askpass_tty(const char *prompt)
         free_prompts(p);
         return NULL;
     } else {
-        char *passphrase = dupstr(p->prompts[0]->result);
+        char *passphrase = prompt_get_result(p->prompts[0]);
         free_prompts(p);
         return passphrase;
     }