Better validate serve_listeners handling of weird edge cases

Looking at https://github.com/twisted/twisted/pull/996/ led me to investigate more the difference between EMFILE and WSAEMFILE... the latter is the special "winsockets" version of EMFILE, and apparently they are actually different:

In [2]: errno.EMFILE
Out[2]: 24

In [3]: errno.WSAEMFILE
Out[3]: 10024

And right now serve_listeners has special handling for EMFILE, but not WSAEMFILE... so it's probably broken.

We should:

• Add the WSA variants to the list of errnos here: https://github.com/python-trio/trio/blob/7b7b7d1431644233a2c06928802be6cdca2e35c6/trio/_highlevel_serve_listeners.py#L11-L16
• Double-check if we need to modify the list in SocketListener: https://github.com/python-trio/trio/blob/7b7b7d1431644233a2c06928802be6cdca2e35c6/trio/_highlevel_socket.py#L291-L311
• Modify our EMFILE test so that it actually provokes a real EMFILE, instead of a fake one like we do now: https://github.com/python-trio/trio/blob/7b7b7d1431644233a2c06928802be6cdca2e35c6/trio/tests/test_highlevel_serve_listeners.py#L102-L105

Start with the errno handling in trio/_highlevel_serve_listeners.py and compare it with SocketListener in trio/_highlevel_socket.py. Review test_highlevel_serve_listeners.py around the EMFILE test, then verify that the relevant Windows errno variants and a real EMFILE condition are covered by the tests.