Fix bugs:

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 :)

Modules/posixmodule.c

@@ -3285,9 +3285,15 @@ _PyPopenCreateProcess(char *cmdstring,
 
 			s2 = (char *)_alloca(x);
 			ZeroMemory(s2, x);
+			/* To maintain correct argument passing semantics,
+			   we pass the command-line as it stands, and allow
+			   quoting to be applied.  w9xpopen.exe will then
+			   use its argv vector, and re-quote the necessary
+			   args for the ultimate child process.
+			*/
 			PyOS_snprintf(
 				s2, x,
-				"%s \"%s%s%s\"",
+				"\"%s\" %s%s%s",
 				modulepath,
 				s1,
 				s3,