linux: always deregister closing fds from epoll

If the same file description is open in two different processes, then
closing the file descriptor is not sufficient to deregister it from the
epoll instance (as described in epoll(7)), resulting in spurious events
that cause the event loop to spin repeatedly. So always explicitly
deregister it.

Fixes #1099.
unix, windows: add uv_loop_init and uv_loop_close

These functions supersede uv_loop_new and uv_loop_delete.
uv_loop_init initialized a user allocated loop and uv_loop_close
removes all associated resources a loop uses after it has finished
execution.

uv_loop_new and uv_loop_delete are now deprecated.