ensure closing the filedescriptor after stopping the iomtee
allow to access the iomux from multiple threads if necessary

note that when in thread-safe mode the iomux can be used from multiple threads
an filedescriptors and timers can be added/removed concurrently.
The only exceptions are calls to iomux_run()/iomux_loop() which still need
to be serialized to avoid handling the same events on multiple threads
(calls to select()/epoll_wait()/kqueue() are not protected by a mutex in the actual implementation)