test__threadpool.py: avoid spurios 'switched unexpectedly' messages because if kill() that switched out

It was previously possible to do the same by removing the source
repositories. But that's not easy to do when installing with pip.

The following command now installs without the embedded libraries:

,----
| gevent_embed=no pip install gevent
`----

the codespeak.net page is a bit outdated.

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