bpo-40334: Rename PyConfig.use_peg to _use_peg_parser (GH-19670)

* Rename PyConfig.use_peg to _use_peg_parser
* Document PyConfig._use_peg_parser and mark it a deprecated
* Mark -X oldparser option and PYTHONOLDPARSER env var as deprecated
  in the documentation.
* Add use_old_parser() and skip_if_new_parser() to test.support
* Remove sys.flags.use_peg: use_old_parser() uses
  _testinternalcapi.get_configs() instead.
* Enhance test_embed tests
* subprocess._args_from_interpreter_flags() copies -X oldparser
diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 6539596..8326c23 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -149,7 +149,7 @@
 
     /* Enable PEG parser?
        1 by default, set to 0 by -X oldparser and PYTHONOLDPARSER */
-    int use_peg;
+    int _use_peg_parser;
 
     /* Enable tracemalloc?
        Set by -X tracemalloc=N and PYTHONTRACEMALLOC. -1 means unset */