Add WL_SOCKET_ACCEPT event to WaitEventSet API.

Add WL_SOCKET_ACCEPT event to WaitEventSet API.

To be able to handle incoming connections on a server socket with
the WaitEventSet API, we'll need a new kind of event to indicate that
the the socket is ready to accept a connection.

On Unix, it's just the same as WL_SOCKET_READABLE, but on Windows there
is a different underlying kernel event that we need to map our
abstraction to.

No user yet, but a proposed patch would use this.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com