core.ppyx: fix compilation on windows where 'child' watchers are not available; add 'ref' argument to child() function of loop

it used to raise an error while it should return None

core: add child handler. WARNING: libev reaps all child processes now. to disable it, pass 'nochild' option to loop.

remove baseserver.DEFAULT_MAX_ACCEPT variable; add get_listener() class methods on StreamServer and DatagramServer

it was not enabled in patched tests because monkey.patch_all() imports threading and that prevents Hub from being patched with CountingHub

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