Convert share/examples/Makefile over to using FILES and FILESDIR.

The goal is to avoid using install directly so we can make changes the affect
how the entire system is installed, without having to touch many places.

This is part of the packaging base work.

Reviewed by: will
Approved by: bapt (mentor), allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D1513