Main: Optimize calls to epoll_ctl()

Main: Optimize calls to epoll_ctl()

It often happens that a file callback is removed and then reinstated
again before the next main loop iteration (e.g., on a boundary between
two blocks if block_io is used). This caused lots of unnecessary
calls to epoll_ctl().

We now put all files on which file_chg() was called to a separate list
and just before we call epoll_wait(), we process this list and check
which files have changed their event mask since the last iteration.