make Pool's add() and discard methods usable on their own. Based on pull request #10 (https://bitbucket.org/denis/gevent/pull-request/10/fix-2-bugs-in-pool-class) by Danil Eremeev.

core: always pass EVFLAG_NOENV to libev's loop. LIBEV_FLAGS env var is now ignored (use GEVENT_BACKEND)

python's threading module relies on the lock's class __exit__ to not
call into the tracing function set via sys.settrace until the lock
really is released. The only way to prevent this is to implement it in
C. So, we move coros.Semaphore to a cython module.

fixes the failing tests in test__threading_vs_settrace.py

explicitly test that sys.settrace isn't called and test the same
issue when a non-main thread exits.

i.e. rename setup and test_main, which are otherwise used by py.test
as a setup/test function.

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.