kqueue API: filter flags

• EV_ADD, EV_DELETE: put and remove a filter

• EV_ENABLE, EV_DISABLE: activate and disactivate a filter

• EV_ONESHOT: when event occurs, remove filter

• EV_CLEAR: reset event when user retrieves it

• EV_EOF: set by filters to report EOF alike conditions

• EV_ERROR: set by filters to report error conditions

Notes: