Patch by Vajrasky Kok.

to stdin within subprocess.Popen.communicate.  5-10% less cpu usage.

error upon client disconnection.

(grafted from c80083ad142db2939507800c755082293a87f2de)

descriptor 0, 1 or 2 is closed.
The errpipe_write fd will always be >= 3.

Patch by Daniel Urban.

causing test_multiprocessing_forkserver and test_multiprocessing_spawn
failures on head (3.4).

descriptor 0, 1 or 2 is closed.

This fix does not cause any degradation in performance.

Based on patches by Christian Heimes and Vajrasky Kok

This test requires the ifconfig executable on $PATH, /sbin/, or /usr/sbin.

pythoncore sub-project.  This should prevent build errors due a previous
build's python(_d).exe still running.