* 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()
Start to split regrtest.py into smaller parts with the creation of
Lib/test/libregrtest/cmdline.py: code to handle the command line, especially
parsing command line arguments. This part of the code is tested by
test_regrtest.