net/nsscache: add port

nsscache is a utility for caching directory service information into local
cache files.

On FreeBSD, it is intended to be used with the net/libnss-cache port but it
can also output BDB files for use by other operating systems.

WWW: https://github.com/google/nsscache

PR: 200123
Submitted by: Kevin Bowling <kevin.bowling@kev009.com>
Obtained from: Limelight Networks