spawn manager greenlet if necessary to finish off excess threads

test__greenness.py: import greentest before monkey patching, since patch_all() installs hub and greentest monkey patches Hub class

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