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.)

mininet/net.py

@@ -109,7 +109,7 @@ from mininet.util import ( quietRun, fixLimits, numCores, ensureRoot,
 from mininet.term import cleanUpScreens, makeTerms
 
 # Mininet version: should be consistent with README and LICENSE
-VERSION = "2.3.1b3"
+VERSION = "2.3.1b4"
 
 class Mininet( object ):
     "Network emulation with hosts spawned in network namespaces."