postgresql plugin: Define default queries in a configuration file.

postgresql plugin: Define default queries in a configuration file.

Instead of writing custom code for each default query, those are now defined
in a configuration file which is parsed using the same mechanism as used for
user-defined queries. This configuration file (postgresql_default.conf) is
installed into $pkgdatadir.

Querying the pg_stat_database view is not currently supported this way. This
would require support for parameters in user-defined queries which is not
(yet) available.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>