Part way to allowing "from __future__ import generators" to communicate

that info to code dynamically compiled *by* code compiled with generators
enabled.  Doesn't yet work because there's still no way to tell the parser
that "yield" is OK (unlike nested_scopes, the parser has its fingers in
this too).
Replaced PyEval_GetNestedScopes by a more-general
PyEval_MergeCompilerFlags.  Perhaps I should not have?  I doubted it was
*intended* to be part of the public API, so just did.

Modules/main.c

@@ -298,7 +298,7 @@ Py_Main(int argc, char **argv)
 			Py_DECREF(v);
 	}
 
-	cf.cf_nested_scopes = 0;
+	cf.cf_flags = 0;
 
 	if (command) {
 		sts = PyRun_SimpleString(command) != 0;