postgresql driver: escape backslashes only when server requires it

Task-number: QTBUG-30076

Change-Id: I408cda941884f01484d0edfa82c91fc19cb8718c
Reviewed-by: Israel Lins Albuquerque <israelins85@yahoo.com.br>
Reviewed-by: Sergey Blagodarin
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Andras Mantia <andras@kdab.com>
(cherry picked from qtbase/e3c5351d06ce8a12f035cd0627356bc64d8c334a)
Reviewed-by: Mark Brand <mabrand@mabrand.nl>