Richer data type for interactive prompt results.

All the seat functions that request an interactive prompt of some kind
to the user - both the main seat_get_userpass_input and the various
confirmation dialogs for things like host keys - were using a simple
int return value, with the general semantics of 0 = "fail", 1 =
"proceed" (and in the case of seat_get_userpass_input, answers to the
prompts were provided), and -1 = "request in progress, wait for a
callback".

In this commit I change all those functions' return types to a new
struct called SeatPromptResult, whose primary field is an enum
replacing those simple integer values.

The main purpose is that the enum has not three but _four_ values: the
"fail" result has been split into 'user abort' and 'software abort'.
The distinction is that a user abort occurs as a result of an
interactive UI action, such as the user clicking 'cancel' in a dialog
box or hitting ^D or ^C at a terminal password prompt - and therefore,
there's no need to display an error message telling the user that the
interactive operation has failed, because the user already knows,
because they _did_ it. 'Software abort' is from any other cause, where
PuTTY is the first to know there was a problem, and has to tell the
user.

We already had this 'user abort' vs 'software abort' distinction in
other parts of the code - the SSH backend has separate termination
functions which protocol layers can call. But we assumed that any
failure from an interactive prompt request fell into the 'user abort'
category, which is not true. A couple of examples: if you configure a
host key fingerprint in your saved session via the SSH > Host keys
pane, and the server presents a host key that doesn't match it, then
verify_ssh_host_key would report that the user had aborted the
connection, and feel no need to tell the user what had gone wrong!
Similarly, if a password provided on the command line was not
accepted, then (after I fixed the semantics of that in the previous
commit) the same wrong handling would occur.

So now, those Seat prompt functions too can communicate whether the
user or the software originated a connection abort. And in the latter
case, we also provide an error message to present to the user. Result:
in those two example cases (and others), error messages should no
longer go missing.

Implementation note: to avoid the hassle of having the error message
in a SeatPromptResult being a dynamically allocated string (and hence,
every recipient of one must always check whether it's non-NULL and
free it on every exit path, plus being careful about copying the
struct around), I've instead arranged that the structure contains a
function pointer and a couple of parameters, so that the string form
of the message can be constructed on demand. That way, the only users
who need to free it are the ones who actually _asked_ for it in the
first place, which is a much smaller set.

(This is one of the rare occasions that I regret not having C++'s
extra features available in this code base - a unique_ptr or
shared_ptr to a string would have been just the thing here, and the
compiler would have done all the hard work for me of remembering where
to insert the frees!)

ssh/common.c

@@ -845,15 +845,13 @@ bool ssh2_bpp_check_unimplemented(BinaryPacketProtocol *bpp, PktIn *pktin)
  * host key is already known. If so, it returns success on its own
  * account; otherwise, it calls out to the Seat to give an interactive
  * prompt (the nature of which varies depending on the Seat itself).
- *
- * Return values are 0 for 'abort connection', 1 for 'ok, carry on',
- * and negative for 'answer not received yet, wait for a callback'.
  */
 
-int verify_ssh_host_key(
+SeatPromptResult verify_ssh_host_key(
     InteractionReadySeat iseat, Conf *conf, const char *host, int port,
     ssh_key *key, const char *keytype, char *keystr, const char *keydisp,
-    char **fingerprints, void (*callback)(void *ctx, int result), void *ctx)
+    char **fingerprints, void (*callback)(void *ctx, SeatPromptResult result),
+    void *ctx)
 {
     /*
      * First, check if the Conf includes a manual specification of the
@@ -878,7 +876,7 @@ int verify_ssh_host_key(
                 fingerprint = p ? p+1 : fingerprint;
                 if (conf_get_str_str_opt(conf, CONF_ssh_manual_hostkeys,
                                          fingerprint))
-                    return 1;                  /* success */
+                    return SPR_OK;
             }
         }
 
@@ -902,12 +900,12 @@ int verify_ssh_host_key(
             if (conf_get_str_str_opt(conf, CONF_ssh_manual_hostkeys,
                                      base64blob)) {
                 sfree(base64blob);
-                return 1;                  /* success */
+                return SPR_OK;
             }
             sfree(base64blob);
         }
 
-        return 0;
+        return SPR_SW_ABORT("Host key not in manually configured list");
     }
 
     /*
@@ -915,7 +913,7 @@ int verify_ssh_host_key(
      */
     int storage_status = check_stored_host_key(host, port, keytype, keystr);
     if (storage_status == 0) /* matching key was found in the cache */
-        return 1;            /* success */
+        return SPR_OK;
 
     /*
      * The key is either missing from the cache, or does not match.
@@ -994,3 +992,22 @@ void ssh1_compute_session_id(
     put_data(hash, cookie, 8);
     ssh_hash_final(hash, session_id);
 }
+
+/* ----------------------------------------------------------------------
+ * Wrapper function to handle the abort-connection modes of a
+ * SeatPromptResult without a lot of verbiage at every call site.
+ *
+ * Can become ssh_sw_abort or ssh_user_close, depending on the kind of
+ * negative SeatPromptResult.
+ */
+void ssh_spr_close(Ssh *ssh, SeatPromptResult spr, const char *context)
+{
+    if (spr.kind == SPRK_USER_ABORT) {
+        ssh_user_close(ssh, "User aborted at %s", context);
+    } else {
+        assert(spr.kind == SPRK_SW_ABORT);
+        char *err = spr_get_error_message(spr);
+        ssh_sw_abort(ssh, "%s", err);
+        sfree(err);
+    }
+}