New Port: dns/dnscrypt-wrapper

This is a port of dnscrypt-wrapper, which adds dnscrypt support to any name
resolver. It is the server-side counterpart of dnscrypt-proxy, and is in
fact derived from its source.

PR: 200015
Submitted by: freebsd@toyingwithfate.com
Approved by: feld (mentor)
Differential Revision: https://reviews.freebsd.org/D3535