Add new port dns/dnstable.

dnstable implements an encoding format for passive DNS data. It
consists of a C library, libdnstable, and several command line
utilities for creating, querying, and merging dnstable data files.

It stores key-value records in Sorted String Table (SSTable) files
and provides high-level interfaces for querying or iterating over
the stored records. dnstable encodes individual records using a
format tailored for efficiently storing passive DNS data and can
quickly perform both "forward" and "inverse" searches.

Differential Revision: https://reviews.freebsd.org/D2214
Approved by: mat (mentor)
Sponsored by: Farsight Security, Inc.