Postgis provider: support for PostgreSQL service connection parameter

[neumann]

It would be nice if the PostgreSQL provider would accept the service parameter alternatively to hostname/port (and potentially database)

This would allow an easier change the underlying postgresql server (e.g. in case of a crash or new server installation) since one would connect to the service rather than a hostname and port.

This is also important in case of a db-replication and switchover.

The provider connection dialogue should offer an additional line named "service" and, if the service value is present, it would have precedence over host and port.

[neumann]

see also ​http://www.postgresql.org/docs/current/static/libpq-pgservice.html

example pg_service.conf:

#zeigt auf den jeweils aktiven master server, bzw. webserver #bei switchover hostname anpassen [pg_uster] host=srv138 port=5432 dbname=uster

[jef]

implemented in r15258.