API refactoring: separand handles for the rules and head



<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/yandex-sysmon/powny/180)
<!-- Reviewable:end -->

do not use `the` with plural word form (with `s` at the end)