2.3.1b4 (#1184)

Including changes to mnexec -c to reduce the number of
close() system calls, which may help with performance in
containers on Ubuntu 22.04.

Note that systemd-udevd also seems to be a cause of performance issues
and should probably be disabled while running Mininet.

Adding dbeug logging to test_simpleperf because it is failing
intermittently. (Note with packet loss it can fail, but this
is not how we should be testing it.)

README.md

@@ -2,7 +2,7 @@ Mininet: Rapid Prototyping for Software Defined Networks
 ========================================================
 *The best way to emulate almost any network on your laptop!*
 
-Mininet 2.3.1b3
+Mininet 2.3.1b4
 
 [![Build Status][1]](https://github.com/mininet/mininet/actions)
 
@@ -70,7 +70,7 @@ Mininet includes:
 
 ### Python 3 Support
 
-- Mininet 2.3.1b3 supports Python 3 and Python 2
+- Mininet 2.3.1b4 supports Python 3 and Python 2
 
 - You can install both the Python 3 and Python 2 versions of
 Mininet side by side, but the most recent installation will