pg_service.conf no longer searched for in a good default

[benchub]

In versions <= 1.5.9, pg_service.conf is searched for in ~/, followed by /etc/postgres-common/.

In newer versions, the fallback for ~/ is /home/lars/comcard/ruby-pg/ports/aarch64-linux/etc/postgresql/. That is less than ideal. :)

[larskanis]

With #668 the fallback path is /etc/postgresql/ which is different to the Ubuntu's path /etc/postgres-common/, but which is the PostgreSQL default path. Since the native gems are cross-distribution, we can not easily use a distribution specific path.