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.