Dns bind changes (#1392)

* `private_dns_server_using_bind.md` changes
* sentence style capitalization on headings
* remove all of the `iptables` references
* move `firewalld` rules out of the individual (8 and 9) sections so that these are global for both versions
* remove most passive voice
* replace most "we" with "you" and replace others with "the author"
* sentence simplification throughout

* Forgot to save my last changes