popen4(), class Popen4: popen4() support for Unix.
popen2(), popen3(): Reversed order of bufsize and mode parameters to
comply with what was here before (Python 1.5.2).
class Popen3: Factored the __init__() into a more basic initializer and
a helper method, to allow some re-use by the Popen4 class.
Use os.dup2() instead of os.dup() to create the proper
file descriptors in the child process.
This closes SourceForge bug #115330 and partially closes #115353.