core: if default loop was destroyed, make it impossible to request a default loop in the future; also clean syserr callback

this error:
cd debian/python-gevent-dbg/usr/lib/debug/usr/lib && mv pyshared pymodules
cd: 1: can't cd to debian/python-gevent-dbg/usr/lib/debug/usr/lib
make[1]: *** [override_dh_strip] Error

Do not mention gevent.six in changelog, it's not part of public interface and might be removed in the future

- SpawnedLink is no longer mentioned because it's an implementation detail
- __class__.__name__ is no longer used because not all object have __class__

the tests did bail out cause I had a threadpool module somewhere in
sys.path. better be explicit about what to test instead of relying on
the user not having installed modules with conflicting names.

raise TypeError it it's not. the functionality to 'link to a greenlet'
has already been removed. make sure we fail early instead of the error
possibly going unnoticed when one passes a greenlet or anything not
callable.

otherwise py.test executes it as a test function.

testrunner.py otherwise buffers output inside my eshell sessions.

the code was using ex=sys.exc_info() instead of ex=sys.exc_info()[1];
only shows up when sqlite3 is not installed.

cythonpp.py: copy implementations of itertools.combinations() and itertools.product(). This fixes cythonpp.py on python 2.5.

itertools.combinations on snow leopard's python 2.6.1 raises a
ValueError instead of returning an empty iterator.

stupid setuptools creates a pure python load for ares.so. this makes
test__execmodules.py work again.