Flush log file after asynchronous askappend.

When I made the 'overwrite or append log file?' dialog box into a
non-modal one, it exposed a bug in logging.c's handling of an
asynchronous response to askappend(): we queued all the pending log
data and wrote it out to the log file, but forgot the final fflush
that would have made sure it all actually _went_ to the log file. So
one stdio buffer's worth could still be held in the C library, to be
released the next time log data shows up.

Added the missing logflush().

logging.c

@@ -153,6 +153,7 @@ static void logfopen_callback(void *vctx, int mode)
 	logwrite(ctx, data, len);
 	bufchain_consume(&ctx->queue, len);
     }
+    logflush(ctx);
 }
 
 /*