Add support for database properties. (#491)

The Postgres tooling for configuration variable settings that are specific
to a database is only activated when pg_restore --create is used, which
means that pg_restore is in charge of CREATE DATABASE.

In the context of pgcopydb, we do not want pg_restore to create the target
database for us, but we still want to copy over the database specific
settings that have been applied with either of the following commands:

    ALTER DATABASE SET
    ALTER ROLE IN DATABASE SET

To achieve that we need to copy some code from the pg_dump and pg_restore
implementation and along with it we also vendor-in some of the Postgres
common code (a part of string_utils.c) to avoid link-time issues on
different packaging systems.