_PyGILState_NoteThreadState was declared as static inconsistently.
Make it static as it's not necessary outside of this module.

Some tests failed because errno was reset to 0. (I think the tests
that failed were at least: test_fcntl and test_mailbox).
Ensure that errno doesn't change after a call to Py_END_ALLOW_THREADS.
This only affected debug builds.

This needs to be ported to HEAD.  I'll try to remember to do that tomorrow.
(Anyone, feel free to port it.)