key list data structure in the thread startup path.

This change is a companion to r60148 which already successfully dealt with a
similar issue on thread shutdown.

In particular this loop has been observed happening from this call path:
 #0  in find_key ()
 #1  in PyThread_set_key_value ()
 #2  in _PyGILState_NoteThreadState ()
 #3  in PyThreadState_New ()
 #4  in t_bootstrap ()
 #5  in pthread_start_thread ()

I don't know how this happens but it does, *very* rarely.  On more than
one hardware platform.  I have not been able to reproduce it manually.
(A flaky mutex implementation on the system in question is one hypothesis).

As with r60148, the spinning we managed to observe in the wild was due to a
single list element pointing back upon itself.

This should prevent some test_multiprocessing failures

Heavily revised, comprising revisions:
46640 - original trunk revision (backed out in r46655)
46647 - markup fix (backed out in r46655)
46692:46918 merged from branch aimacintyre-sf1454481

branch tested on buildbots (Windows buildbots had problems
not related to these changes).

46640 Patch #1454481:  Make thread stack size runtime tunable.
46647 Markup fix

The first is causing many buildbots to fail test runs, and there
are multiple causes with seemingly no immediate prospects for
repairing them.  See python-dev discussion.

Note that a branch can (and should) be created for resolving these
problems, like

svn copy svn+ssh://svn.python.org/python/trunk -r46640 svn+ssh://svn.python.org/python/branches/NEW_BRANCH

followed by merging rev 46647 to the new branch.

  Itanium2 with HP's 64-bit compiler (SF patch #1225212).

release-build failures noted in bug 1041645.

This is a critical bugfix.  I'm not going to backport it, though (no time).