Misc gc code & comment cleanups. (GH-16752)

* Misc gc code & comment cleanups.

validate_list:  there are two temp flags polluting pointers, but this checked only one.  Now it checks both, and verifies that the list head's pointers are not polluted.

move_unreachable: repaired incoherent comments.  Added new comments.  Cleared the pollution of the unreachable list head's 'next' pointer (it was expedient while the function was running, but there's no excuse for letting this damage survive the function's end).

* Update Modules/gcmodule.c

Co-Authored-By: Pablo Galindo <Pablogsal@gmail.com>