dns: Refactor forEach to map

Refactor a forEach to a `map` in the `setServers` function of the
dns module - simplifying the code. In addition, use more descriptive
variable names and `const` over `var` where possible.

PR-URL: https://github.com/nodejs/node/pull/5803


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>