win32: thread: clean some thread API usage

• replace CRITICAL_SECTIONs by Slim Reader/Write locks which are lighter than critical sections. They don't require a release and even provide a static initializer.
• fix loading the WakeOnAddress function from the proper system DLL
• set the fallback functions in the init function used for that particular case
• remove unsused CONDITION_VARIABLE

Only available since Vista so cannot be backported to 3.0.