Partial fix to [ Bug #128685 ] popen on Win9x isnt smart enough about finding w9xpopen.exe.

"Partial" as the code uses sys.prefix in an attempt to locate 'w9xpopen.exe', but sys.prefix is not set if Python can't find it itself.  So this _still_ fails in Pythonwin, but I am committing the patch for 2 reasons:
* Embedded apps that set sys.prefix or use PYTHONHOME will work
* The exception raised on failure to find the executable is far more obvious
1 file changed