ThreadPool: fix _set_size to stop _fork_watcher if it's not needed; also make sure it does not try to sleep when run inside hub

This is to counter cases where SIGCHLD is not delivered or lost which I've seen happening with test_patched_signal.py

fileobject: use __del__ implemented in Cython so that it cannot be interrupted asynchronously; multiple bugfixes

 - add 'close' argument to SocketAdapter
 - handle EINTR
 - remove _flushlock
 - remove closedfileobject

hib.py: Hub.print_exception() always prints an exception now; handle_error() decides if it must be done

examples/psycopg2_pool.py: make it possible to pass arguments to cursor() in execute() and fetch*() methods