periodically flushes the output buffer so that the sender/receiver
can get started on the files sooner rather than later.
+ - Improved the keep-alive code to handle a long silence between the
+ sender and the receiver that can occur when processing the checksum
+ data for a large file.
+
- Improved the auth-errors that are logged by the daemon to include
some information on why the authorization failed (wrong user,
password mismatch, etc.). (The client-visible message is unchanged.)