Windows implementation of _dbus_platform_rmutex_lock() does not match the expectations of the client code

The unix variant of _dbus_platform_rmutex_lock() is implemented as

void
_dbus_platform_rmutex_lock (DBusRMutex *mutex)
{
  PTHREAD_CHECK ("pthread_mutex_lock", pthread_mutex_lock (&mutex->lock));
}

It asserts in any error case and let client code simple use

_dbus_platform_rmutex_lock (mutex)
/* protected access */
_dbus_platform_rmutex_unlock (mutex)

This behavior is not not implemented in the Windows code:

void
_dbus_platform_rmutex_lock (DBusRMutex *mutex)
{
  WaitForSingleObject ((HANDLE *) mutex, INFINITE);
}

which can lead to threading problems, such as those reported at #368 (closed).