Use native Windows events for the event loop.

Use native Windows events for the event loop.

This commit changes the event loop to use WSAEventSelect() and
WSAWaitForMultipleEvents() on Windows. This paves the way for making the
event loop more flexible on Windows by introducing the required
infrastructure to make the event loop wait on any Win32 event.

This commit only affects the internal implementation of the event
module. Externally visible behavior remains strictly unchanged (for
now).