Cleanup regrtest "main()" function

* Rename libregrtest.main_in_temp_cwd() to libregrtest.main()
* Add regrtest.main_in_temp_cwd() alias to libregrtest.main()
* Move old main_in_temp_cwd() code into libregrtest.Regrtest.main()
* Update multiple scripts to call libregrtest.main()
diff --git a/PC/testpy.py b/PC/testpy.py
index 4ef3d4f..709f35c 100644
--- a/PC/testpy.py
+++ b/PC/testpy.py
@@ -26,5 +26,5 @@
             # Add the "test" directory to PYTHONPATH.
             sys.path = sys.path + [test]
 
-import regrtest # Standard Python tester.
-regrtest.main()
+import libregrtest # Standard Python tester.
+libregrtest.main()