457466: popenx() argument mangling hangs python
 226766: popen('python -c"...."') tends to hang

Fixes argument quoting in w9xpopen.exe for Windows 9x.  w9xpopen.exe
also never attempts to display a MessageBox when not executed
interactively.

Added test_popen() test.  This test currently just executes
"python -c ..." as a child process, and checks that the expected
arguments were all recieved correctly by the child process.  This
test succeeds for me on Win9x, win2k and Linux, and I hope it does
for other popen supported platforms too :)