Docs for service file

Here is documentation and a sample file for the libpq service/PGSERVICE
capabilithy we added in 2000. Seems it was never documented. It allows
a central file, pg_service.conf to act as a central location for service
information.

You do connectdb("service=test") and a lookup is done to see the
connection parameters. The code assume pg_service exists in
install_dir/etc. Not sure if that is the right place, but it is a libpq
config file, so none of the other places seemed to make sense. Ideas?

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