Shout more loudly if we can't open a log file.

A user points out that logging fopen failures to the Event Log is a
bit obscure, and it's possible to proceed for months in the assumption
that your sessions are being correctly logged when in fact the
partition was full or you were aiming them at the wrong directory. Now
we produce output visibly in the PuTTY window.

(cherry picked from commit e162810516)

logging.c

@@ -88,16 +88,19 @@ static void logfopen_callback(void *handle, int mode)
     char buf[256], *event;
     struct tm tm;
     const char *fmode;
+    int shout = FALSE;
 
     if (mode == 0) {
 	ctx->state = L_ERROR;	       /* disable logging */
     } else {
 	fmode = (mode == 1 ? "ab" : "wb");
 	ctx->lgfp = f_open(ctx->currlogfilename, fmode, FALSE);
-	if (ctx->lgfp)
+	if (ctx->lgfp) {
 	    ctx->state = L_OPEN;
-	else
+        } else {
 	    ctx->state = L_ERROR;
+            shout = TRUE;
+        }
     }
 
     if (ctx->state == L_OPEN) {
@@ -119,6 +122,23 @@ static void logfopen_callback(void *handle, int mode)
 		       "unknown"),
 		      filename_to_str(ctx->currlogfilename));
     logevent(ctx->frontend, event);
+    if (shout) {
+        /*
+         * If we failed to open the log file due to filesystem error
+         * (as opposed to user action such as clicking Cancel in the
+         * askappend box), we should log it more prominently. We do
+         * this by sending it to the same place that stderr output
+         * from the main session goes (so, either a console tool's
+         * actual stderr, or a terminal window).
+         *
+         * Of course this is one case in which that policy won't cause
+         * it to turn up embarrassingly in a log file of real server
+         * output, because the whole point is that we haven't managed
+         * to open any such log file :-)
+         */
+        from_backend(ctx->frontend, 1, event, strlen(event));
+        from_backend(ctx->frontend, 1, "\r\n", 2);
+    }
     sfree(event);
 
     /*