net/libnss-cache: Add port.

libnss-cache is a NSS module for reading directory service information for
hosts from an indexed, local disk cache of that directory service.

nsswitch.conf services provided: passwd and group

Add 'nsscache' to the desired service(s) and generate the cache with the
net/nsscache port.

WWW: https://github.com/google/libnss-cache

PR: 200120
Submitted by: Kevin Bowling <k@kev009.com>