ports/68321: [NEW PORT] databases/postgis: PostGIS spatial extensions to PostgreSQL

[Sam Lawrance]

This port does include a few features not in the current port
* obeying PREFIX
* using ${EXAMPLESDIR} for the SQL scripts
* build/install docs
* building lwgeom
* couple of other refinements

I have contacted the maintainer; will see if we can fix it up. Meanwhile
this can still be closed.