Commented endif in eventfd.cc.
Removed unused linux_message_hub_t::notify_t::notifier_thread field.
Made the message hub use only one eventfd per thread (or maybe two per thread, sigh).
Made mock_file_t::writev_async allocate an aligned buffer so that it works properly in all allocators.
Merge branch 'next' into sam_1225
Removed overassertions added recently for debugging purposes.