update gevent/libev_vfd.py: remove GEVENT_VFD_MODE, it's no longer needed. import winsock2.h instead of winsock.h

 - install SIGCHLD lazily when first child watcher started
 - properly call WSARecv
 - log WSARecv's and WSASend's errors;
 - install periodic timer for waitpid()
 - ev_loop_destroy(): do not stop childev watcher when destroing non-default loop

add xtest_signal.py which reproduces SIGCHLD being missed (fixed in https://bitbucket.org/denis/gevent/changeset/adb8b5ac698c)

greentest.py: in dbg mode, call setUp() and tearDown() properly when executing tests multiple times for refcount